to be able to keep this position and even rank higher, my suggestions are:
Update and bring fresh and unique content as often as everyday or 3 times a week
Always take a good note of the keyword density in each page in order to let google know you really deserve to rank for this term
Have you already created a sitemap? If yes good, If NO- then make one
Do not overdo submission since you are already in rank. Never piss the SE
Ping your sites after each update - or just leave it if you notice a sudden aggressiveness of the spiders(check ur awstats)
Add pages as much as 5-10 a day and no more than 20
Evaluate some of your weak pages and reconstruct all its meta tags in order to support the ones in rank
Regularly submit your site to related and of good reputation(PR) directories - this will strengthen your rep making google think youre an authority site already. Are you already listed in DMOZ? If not, dont wait any longer, submit it
Last, regularly check your competitors and see what they are doing - learn from that