Random thoughts from when I write job reqs:

Make sure there is a clear description of the job itself and what the challenges the company and the role itself are trying to solve ("Looking for Ruby on Rails devs, call us!" doesn't explain what problems you'll be solving, etc). GOOD developers don't look for a new job using X, Y and Z languages, they look for a new job tackling problems A, B, and C.

Don't include requirements that are superfluous, unless you can really justify them. "Must have 5 years experience" - why? I know devs with 2 years experience that will kick the shit out of a 10 year vet. "Must be capable to lead a small dev team" or "Experience with several discrete projects a must to be able to advise from experience on architecture choices" is better.

Ditto with CS degrees: don't mandate one unless there is a good reason.

Be clear yourself with what the role is going to be: are you looking for an engineer or a support guy, dev ops, etc? "Spray and Pray" is not a strategy and looking for that experienced engineer who is fluent with UX and design while also able to administrate the production servers and load balancers is going to net you people who know nothing about anything.

State the current stage + status of the company "pre-funded", "series a funded", "profitable". Different people are attracted to different stages (and the risk that goes with them) so be clear in the job req to pre-filter those who are not. An engineer with a wife and two young kids is unlikely to take your job at your pre-funded startup so be clear where you are.

Don't use words like "rockstar", "code-ninja" or guru. You come across looking like a dick - and far worse, that I might actually end up working with someone who really thinks and behaves like he is a "rockstar" because he was hired as one.

Be clear with your methodologies, coding practices (pair programming, peer code review, etc) and anything else that will make you look attractive. Consider getting some before you post your ad if you don't currently have any (why do I want to go work in chaos?).

State where you are located, and not just "San Francisco" (for example). Mention the neighborhood or even the building. People living in East Bay are going to want to know if they can BART in, South Bayers will want to know how bad the drive will be ONCE they get into the city/if they can CalTrain and even locals are going to turn their nose up at your Outer Sunset location if they're paying through the nose to live in SoMa. I also want to know what kind of community there is around my potential work place ("We're in Twitter's building", "we hang out in South Park most lunchtimes"). Also state if telecommute is an option.

Don't forget to mention benefits (or state if there are none yet) - even geeks in their mid-twenties want to know if you are going to cover their dental. Same for stock, you don't have to mention remuneration specifics but be clear if the remuneration will include stock, and be especially clear if you are looking to offer a package that is "equity heavy" (ie below market $ salary). Again, this will filter out people who are not suitable for that situation and manage expectation of those that enquire.

Include photos of your work place, even if you just have basic digs (which is fine if you are early stage) You can tell a lot about a startup and a job from looking a their offices.

Don't use recruiters. Do I have to explain why?

But my best tip of all:

You/your founders should be spending a large amount of your time curating your network such that you don't need to put out job reqs because you can simply hire from your network.

Also don't be afraid to simply identify developers in existing startups and reach out to them directly - they may not be actively looking but will entertain your interest (and perhaps) subsequent job offer. Even in this economy (in Bay Area, at least) good developers are seldom out of work so be prepared to poach rather than search the job seeker pool.