When's the last time you offered help to another startup? Even if it's just sitting down and listening to someone's pitch for them, or giving product advice, spending a few minutes with someone else's big idea can reap tons of benefits, and not just for them.
First off all, being selfless generates goodwill. Any time you spend on someone else's project will be more than reciprocated when you need something in return. So when you're looking for people to invite their friends and spread the word about your app, don't be surprised when your strongest supporters are the guys you went to lunch with last week to be an elevator pitch sounding board. I think too many people bury themselves in their own work, and then when it comes down to needing a supportive community to grow their service, they find a lot of tumbleweed blowing through their social graph. Don't expect to disappear on your friends and fellow entrepreneurs for months on and and then expect the cavalry to arrive when you need a Digg.
Secondly, pulling your head out of the sand once in a while can inspire you. I make it a point to spend time with entrepreneurs who work in other verticals, because you never know when an idea that works somewhere else can be reapplied in a novel way. I don't spend all of my time in the job space because it contains a lot of the kind of stagnant thinking I want to disrupt with Path 101. Some of my best ideas have come from startups and just other professionals in completely different industries.
A lot of people cling to startups who seem to be on the rise, but fail to be there for others when things aren't going so well. Helping someone who is down and out in a difficult time is not only severely needed, because the ups and downs of the startup world can be difficult, but can also put you in the right place at the right time when companies start scuttling themselves. You might be able to take over a cheap lease, hire your superstar coding buddy who tried to go out on his own but it didn't work, or grab an unwanted server (or two).
Community participation is also important. By sharing your successes and failures with others, not only can that raise your own profile, but contributing to a strong local tech community can have longer term benefits. Maybe it will be easier to hire your next developer down the line because more people will know what you're up to, or the community will just attract more people. I never thought of any of this stuff when I started nextNY, but I can clearly see a positive ROI to my participation. If it wasn't for nextNY, I never would have found my partner Alex, because I caught up with him at a couple of community events right after he left his last job.
I don't know if this makes me sound insincere or not--I'm just trying to point out to those who wouldn't normally take their eyes off their own work that there can be a positive ROI to being a bit selfless. Not everyone is naturally this way, and so sometimes people need to see incentives, which, to me, is fine as long as the help is authentic.