I've been thinking about this and I came to the conclusion that Voldemort was an idiot who didn't deserve to be in Slytherin.

Okay, that might be a but harsh. But considering that he was the top student in Hogwarts, getting straight O's his entire time there, and that he was supposed to be more Slytherin than Salazar himself, he doesn't act very well as a Dark Lord. I can think of a vast number of things a sensible Dark Lord could've done, and ones that a future, more competent Dark Lord might do.

First of all, make sure that your minions are loyal to you. Sure, if they're afraid of crossing you then they won't betray you, but they're also unlikely to offer advice or do much of anything that you don't order them too. Consider Pettigrew, who would never have returned to Voldemort if he had any other option. Be good to your minions, make sure that they know you value their ideas and won't skewer them for disagreeing with you. Of course, that doesn't mean that you should trust them. Take measures to ensure their loyalty which no loyal minion could object to. For example, make it a condition of working with you that you must accept a dormant Imperius, one which activates if and only if the person betrays you. If someone refuses this, they clearly see it as a possibility that they will betray you in the future, so you wouldn't want them anyway. As an added bonus, point out that if you're ever defeated they'll be able to swear under Veritaserum that they were under the Imperius while serving you, and that a magical scan or Veritaserum questioning would agree with this. Protection for both them and you.

Now, the Dark Mark was a good idea in theory, but in practice it was terrible. After all, an easily identifiable and magical tattoo makes finding your agents incredibly easy. The idea of it - a way for Voldemort to summon his followers, a way for them to identify each other, and a way for them to summon him in turn - was great. But in practice? Better ways of doing it. What I'm thinking of is sets of enchanted contact lenses. Not undetectable, but impossible to distinguish from normal contacts; except that they glow when viewed through other contacts. See a person with glowing eyes? He's on your side! Also, there could (for lack of a better word) an IM service with these. Activate the dictation with a codeword you choose and send text messages that will show up on all the other contacts. After all, there are definitely ways to make messages appear. The dictation would be a variant of the charm on a Dictation quill, and it could be displayed with something similar to the charm the memory of Tom Riddle used to write his name in the Chamber of Secrets. After that, a simple (well, not so simple, but you know what I mean) Protean charm would let it show up on everyone else's contacts. You can even let them send messages only to one other person, but it might be a good idea to have them appear on yours anyway - keep an eye on what they're saying.

Make sure your minions are well prepared for contingencies! Equip everyone with a mokeskin pouch (only the owner can get stuff from it) with an undetectable space enlargement charm on it, like Hermione's beaded bag in book seven. In it, they must have at least a full set of medical potions, a set of potions ingredients, a cauldron, and a must have at least one spare wand and two emergency portkeys rings (portkeys can be activated at a specific time, when anyone touches them, or when a codeword is said) with them at all times. The portkeys should be toe rings for the best results, as people are not as likely to check feet for possibly-enchanted jewelry as they are to check hands. Note also that portkeys function through anti-apparation fields (Dumbledore used a Portkey to send Harry from his office to Grimmauld Place in book five), and through Fidelius, although this may require that the maker of the portkey or the person being sent or both knows the secret already. Make sure everyone can cast a quality Disillusionment charm, and give them all practice at seeing through it.

Make good use of the Fidelius charm. Remember, Arthur Weasley was the Secret-Keeper for the Burrow and Bill Weasley was the Secret Keeper for Shell Cottage - there's absolutely no reason for someone not to make themselves the Secret Keeper for their own residence. Make sure everyone's homes are well warded and have a Fidelius over them; have them be the Keepers for their own homes so that the capture of one person can't give everyone away. You'd be in on all the Secrets, of course.

Next is a sort of general thing. If you can do something in a couple different ways to get the same result, do it however is easiest. I'm thinking of how Voldemort spent the better part of a year kidnapping Harry when not only could he have just gotten fake Moody to give Harry a portkey at any point (or even just had him polyjuice into a random muggle and flip Harry a portkeyed knut during a Hogsmeade trip), but he could've used literally any other wizard who still hated him; he could've just used Bertha Jorkins, for example. And letting the Cup still be a portkey back to Hogwarts? Dumb. So do things in the easiest ways that you know will work. Now, I know that Voldemort wouldn't have been immune to Harry's blood protection death-touch if he used someone else's blood, but he didn't.

Horcruxes. Now, I don't think that this is the best method of immortality, especially since the evidence suggests that making them damages your ability to think rationally. But if I was going to make them, they would be much harder to find. Just as a few random examples: set one up with alternating levels of portkey enchantments on it, so that it randomly portkeys itself to one of several different safehouses (all of which are under Fidelius with no-one knowing the Secret but you); have one portkeyed into space, then obliviate yourself of where you sent it; drop one in a volcano; drop one in a deep ocean trench; simply cast the Fidelius charm on one to make its existence a secret and never tell anyone. Also, set them all up with a contingency spell on them when they're destroyed; Fiendfyre, to be specific, and a message to yourself so you know that someone destroyed it.

Finally (and this is a good one), do all of your evil activities polyjuiced as one of your minions, with him poly-juiced as you. Slowly promote the minion who masquerades as you until he's your second in command. Make sure that the world fears and hates you to as great a degree as you can manage. Go out in public without the polyjuice, have your minion begin doing an evil thing, then 'betray your master' and kill him. Message all your other servants that they're to 'wake up' from their Imperius trances; remove the dormant spells from then (you can establish them again later) so they can all be innocent and keep serving you. At this point, you can present yourself as a servant of the Light who infiltrated the Dark Lord's organization, gained his trust, and then killed him. You can now basically be Albus Dumbledore.

Only, you know, secretly evil.

As you're in power over a generation or two, you can raise people to believe in you absolutely (especially if you become a Hogwarts Professor; with your beloved status, a little undetectable poison in the Headmaster's tea will get you the position), and control the wizarding world without them ever believing that you're actually evil.

But of course, you are.