[These are my thoughts, not necessarily my employer's. And check out Steve Yegge's tips for a longer but better-written piece.]
Your resume fits on a page. "But I need more than one page--" No, you don't. Either you're junior enough that you can fit it on a page, or you're awesome enough that you don't even need to list every detail. (Exception: researchers. Then you're working more on a CV than a resume.) Yes, this will mean cutting down on stuff. And as you get older, it will mean cutting stuff out. Your awesome responsibilities as a lifeguard at the pool were important when you were a sophomore, not when you're a 30 year-old.
Go re-read your resume with that in mind.
Yes, you need the keywords to be in there. Tell me you were a software engineer. Tell me what languages you know. But you don't need to tell me that a software engineer "designs, implements, and verifies software code to" yadda yadda.
Here's where I diverge most from Steve's advice. There are two uses of the resume. One is to get the interview. The next is to make the interview better. For the first part, you want to trim the fat. For the second, you want to leave what I call "hooks". Interesting sentences that will make your interviewer look down at your resume, then look up at you and say, "huh, this is interesting." Then, it's your time to shine. On my resume, I have hooks for small talk ("Opened for The Who"), and then hooks for what I care about ("enjoy tackling low-hanging technical fruit to bring about societal change").
My resume screener won't care about these hooks. And they probably won't even come up on a first-pass interview (where the interviewer cares more if I've ever actually seen a computer). But when I'm talking with someone who knows I can code, and wants to know if I can code the right things, this is a useful sentence. Put enough of yourself on the page so an interviewer will give you the chance to come out in person.
OK, you have a resume. Now go back and fix it, cause it's probably still bad. No, seriously, 15 minutes now will get you farther faster. Basically, candidates write what they think resume readers want to see. Stop that. Write what you'd want to read.
Steve (linked above, also here) spends a lot of time on the bad writing that pervades many resumes. Go read that. I'll wait. Back? Good. Now, take out your resume, reread each word, and cut it out unless you can convince your antagonistic alter ego that it's truly necessary.
The best way to get a better resume is to ask the 5 people you most admire (in a similar job/field) to see their resumes. You'll probably go "oooh, that looks nice". Then stop and think about what makes it nice. Copy those attributes shamelessly. Then, the next time a friend asks for resume help, say, "okay, and here's mine, so you can see what I like in resumes." (This also has the side benefit of making you keep yours up-to-date)