Since a comment is too short to put my rant, I'll put it here.
The Difficulty of Cheating
Cheating is much too difficult to be worth the trouble. First, you'd have to have all these different, unassociated, valid ways to sign up. That already takes too much time. Second, you'd have to build up enough reputation with ALL of those accounts to be able to upvote/downvote. You might think it's easy - just have the second user upvote the first. However, if both of these dummy accounts have too low of a rep, how do you do that? Third, you have to maintain their reputations. If something causes their rep to drop too low (yes, this can happen), then you have to go through the trouble of further maintenance, especially if you also plan to use these puppets to vote people down.
Now that the "setup" is out of the way, let's keep in mind that there's a 200 daily reputation gain limit. That means you'd only get 200 points a day. Even if you answered 5 questions and got 10 upvotes on each answer, you'd still get only 200 points instead of 500. Let's say you're paranoid and want to ensure that limit, so you set up 20 dummy accounts and get them all to a high enough reputation to be able to vote.
Let's take a worst-case scenario approach, and slowly get it to the best case scenario and what you'd have to do to achieve and maintain that scenario (I use "you" to refer to the generic user, not to you personally). I'll even start with a better than worst case scenario by saying your primary user has ~50 rep to start with.
You begin the setup of your puppet army. Your 20 puppets ask questions, and you vote them up. Supposing that you want the "fast" way to gain rep, you don't make these questions particularly good quality. The community votes these questions down and votes to close them. Well, there goes that rep. So now you have to take the time to make sure each puppet makes contributions of a high enough quality to maintain a good reputation.
OK, so you've got the dummy accounts up to a good enough reputation, which took you a lot of time considering that you need to use 20 different browser sessions or log out and back in a number of times. You also took the time to go through and vote those questions up with your main user. Now, not recognizing that there's a 200 rep limit per day, you go out and answer a bunch of questions, possibly including those asked by your puppets. The delay between question and answer would grow as you go through because you take the time to post a slightly relevant answer to each. Now for the voting with your puppets. You go in and vote your answers up. This took you hours. What's this? Your reputation didn't go up by more than 200? You should have gotten at least 1000. Well, now you realize that you can only earn 100 rep per day. Sure, if you get up to 200 rep, you'll get an association bonus, which can help you a bit, but that's a one-time bonus.
OK, so now you know there's a daily rep limit. This time you either find a question to answer, or you ask your own (using the main or a puppet account). Now, you have to use a different browser session OR log out and log back in to answer the question "first". So you manage to get the first answer, and the answer is garbage. Knowing that nobody would upvote that, you have 20 other qualified accounts (with the setup complete) vote that answer up. The rest of the community will very likely downvote the answer, sometimes to oblivion. This means you have to take time to make the answer at least adequate so that it doesn't get voted down. (this becomes useless later on).
So you take the time to make the answer adequate enough to not be voted down, but not necessarily good enough to be voted up. You have your 20 puppets vote it up. Meanwhile, another user sees the question, has a better answer than you, so he posts it. Other people, including the asker, see that answer, and they vote it up. Sure, you have the accounts to vote it back down, but that would waste their precious reputation. You do it, and - oh no! - one or more of your puppets no longer has enough reputation to vote down, and maybe even some don't have the ability to vote up! Now you realize that you need to manage the puppets' reputations as well as your own since voting down costs reputation.
OK, so you start to rethink managing 20 puppet accounts. You just undo as many as the downvotes as possible (there's a time limit for undoing that) to fix it for now. Now you're back to someone having a better answer than yours. So you have to do more than make your answer (or question) merely adequate; you have to make it good enough to make others want to vote it up.
OK, so you've done all that, and you're still running ragged because of the 20 puppets. You thin the numbers down a bit - after all, your answers (and possibly questions) are good enough that you don't need as many anymore. You also notice that you're running out of material. Uh-oh. You could have your 20 puppets delete their questions and then just ask them yourself, but people would notice, and that would mean trouble. Not wanting to learn how to make a time machine to stop yourself from doing that, you decide not to. You realize that you have to come up with high-quality, original material.
You begin to do this, and you use maybe only 5 of your bots now. You now aren't reaching that 200 point limit. Oh well. Rep is rep. Your questions and answers are starting to be voted up by others anyway. Wait, did I just say that? Other people are voting your quality answers up, and sometimes even giving you a bonus (and immunity to point loss by your answer being voted down) by selecting your answer as the accepted answer? You didn't need your puppets this whole time?
You never needed those puppets
Well, you're all dejected now, because you just wasted hours, if not days, learning these hard lessons about gaming the system. But since you have moved on, you think you're fine. Meanwhile, an investigation of your activities has been going on, and they discover that you've been sock puppeting. Oh dear, you're suddenly banned! No! All that work wasted! You'd have to create a whole new account (which would also be wrong) and start answering legitimately! Looks like it would have been better to answer legitimately from the start.
Given all these requirements, I find it a great insult to our intelligence when you say that we would even think we could ever make it through all that. Even with better conditions, you're bound to be caught, and you're still in for a ton of work. You might as well spend the same amount of time actually doing research and putting up good, well thought out questions.