Have achived #1 & #2 for all of my chosen keywords across Google, MSN & Yahoo, so am naturally pretty chuffed. Now how to stay there?
It will be hard to keep that position in major three SE at the same time. But try to analyze your recent activity and focus on it.
Once you get to number 1 its a question of building and maintaining useful and relevant content. You clearly have enough links but you should still aim to build a small amount of natural links each month. My tactic is to diversify and try to get loads of high rankings across hundreds of uncompetitive terms (the 'long tail' of search). This gives you something to fall back on if you lose the number 1 spot and also helps establish your site as a wider resource in your niche.
As mentioned, build more content, look deeper at the long tail. Also use the adwords keyword tool to find more phrases. Look at capturing visitor's emails, and other ways of increasing the return from those domains. Start more sites on more topics, using a similar approach to what worked previously. Something people miss sometimes when they hit #1 is the #2 spot or the indented result... so good point to mention you have that too.