For me personally it's a few things.
1. Passion. It's not always easy to be passionate about what you do. SEO, design, writing, etc can be pretty damn boring. I try to stick to what I enjoy and pay others to do the rest. (I like some forms of writing, I like networking with people and I like to pretend I know how to program.)
2. I have 3-4 specific goals every day that I force myself to do before doing anything else.
3. I set specific times to work, usually starting after a shower and lasting anywhere from 3-4 hours, usually longer, depending on what I have going on. But when I'm done for the day, I walk away.
4. I reflect on what I've done every day. I feel like shit on days I don't do shit.
5. For me personally, one of my biggest productivity killers is bars. I LOVE to go out and have fun. But anytime I have a hangover I'm worthless. So I keep myself in check. I guess know your weaknesses and attack them is what I'm saying.
6. Agreed with don't jack off. It drains your energy and also drives down your desire to hook up with real chicks (if you're single).
7. I work in 33 minute bursts. Heard it from Eugene Schwartz originally. But I set a timer, go balls to the wall for 33 minutes and then take a 15 minute break.
8. I like to test myself. Like "I'm going to earn $x amount in 2 hours" or "I'm earning enough to cover the rent today".
9. I reward myself. If I get more accomplished than I had planned, I'll do something fun.
10. When I am working, it's at 100% full stop. When I'm in the zone I swear I can get more done in an hour than I can in a day when I'm not feeling it.
11. I like having my back against a wall. I purposely make myself uncomfortable.
12. On your own shit, a good trick is to buy advertising for it in advance if possible. Then it's like a client project. You miss the deadline you fuck yourself out of money.
13. I primarily freelance for clients, and really feel a personal responsibility to do my best and make the deadline. Being accountable to other people is a big motivator for me.
14. When you're not moving it's hard to start. I'll tell myself "I'm just going to email this client" or "I'm just going to write 200 words" and once you get yourself in motion, it's really hard to stop.
15. Forums, FB, Skype, Email, AIM, etc are all HUGE time sucks. All necessary too. But I have times of the day where I'm unavailable on all fronts and focusing on production.
16. Take a vacation. I just did for 2 months. Just started working again a few days ago and I feel really motivated/recharged. I also take 3-4 day "mini-vacations" every couple of months, and make it a point to get away on weekends. Gotta take time to enjoy life.
And it's something I'm always working at.
Staying productive ain't easy. But it's essential if you want to be self-employed and successful.