Keep it simple...
Every project should be something that you can summarize in a single sentence that gives a decent idea of what the project will entail. "My character will be a ranged class that never uses a ranged weapon." "My character will never equip weapons or armor." "I am going to roleplay for one month in a new game with a scheduled ending." "We're going to roleplay two characters that are always together."
All of those sentences leave some questions about what the project will entail, but the gist of it is clear right there. You get the idea very quickly. This has two advantages, the first being that it's easy to explain to someone the bare-bones version of what you're doing. The second is that you have an easy time measuring your actions against a single standard -- if you're unsure if you're breaking a rule of your project, you can go ahead and check the new rule or lack thereof against that central axiom.
...but have more rules
Let's say you're playing a character who is supposedly shunned by animals. That means that in both gameplay and roleplaying, you need to have a list of things to do and a list of things not to do. The character in question cannot ever have a mount or form of transportation that isn't mechanical. He can't use any forms of transport that require same. He should be uncomfortable around animals belonging to other people and possibly dislike those who do have animal companions. There are a lot of restrictions, most of which will make both your play and your interactions with others a bit more difficult.
Having a single sentence is a good start, but you should lay down your rules early, preferably in some form of handy reference for yourself. You need to check against these restrictions to see if you're following them, and while they won't cover every corner case (something will no doubt crop up that you wouldn't have guessed at), they'll cover a lot.
Restrict some things, open others
If your project is to play in a game for a fixed period of time and then stop, obviously you're pretty restricted in how much you can do. But at the same time, you also have some new freedoms. You don't have to worry about killing off a high-level character, because that character is going away anyway. You can make major life changes to characters and have them stick because the whole project has a fixed expiration date. You're barred from some things, but the project also opens up new avenues that you wouldn't explore otherwise.
A purely restrictive project like playing a mute character is generally less fun. It doesn't give you new areas to explore, it just closes off the usual range of basic functionality. Fun projects are usually more based upon closing off some routes and opening others, keeping yourself engaged by doing something notably different. You're still making life a little harder on yourself -- that's part of the point, after all -- but you're not making it unpleasant to log on.
Have a point when you're done
Open-ended projects are problematic. You can argue that all roleplaying is essentially an open-ended project, but when you're creating a set of restrictions to try out something new, you really want to have defined limits. If you have a single character that you're working with, that works. If you've got a time limit or a level range, that works. But you need to be able to say that even if the project doesn't work out, you can still take a step away and do something different.
Of course, it's also relevant to consider whether you're working on a gameplay project that you're trying to roleplay or a roleplaying project that has gameplay implications. Both are perfectly valid ways to start a project, but they're going to lead down different routes. The former means coming up with a set of play restrictions that suggests an interesting set of character traits, where your main goal is to see if you can level and play while adhering to certain limitations. The latter means that your real goal is to have an interesting character who deviates from the norm, and as a result you are playing the game a bit differently.
Make it something worth doing
The whole reason that you're starting a new project is to breathe new life into an old and familiar game, right? So you'd best make sure that you're going to actually breathe in some new life. Otherwise, you're just calling something a project when you're playing the same game as ever and roleplaying to your heart's content.
Which actually still isn't all that bad when you put it like that.
Feedback is welcome, as always, via mail to firstname.lastname@example.org or in the comments below. Next week -- what do you do when you realize that you are the problem?
Every Friday, Eliot Lefebvre fills a column up with excellent advice on investing money, writing award-winning novels, and being elected to public office. Then he removes all of that, and you're left with Storyboard, which focuses on roleplaying in MMOs. It won't help you get elected, but it will help you pretend you did.