hi,
i am a new IRTA at the NIH, as well. here is the way i found a position relatively quickly.
first, it helps if there is a specific area of science that really interests you. i was interested in malaria research, so i tailored my resume and cover letter to reflect that. fill out the online application form and get those reccommendations in ASAP!
next, knowing what you might be interested in, go the the NIH's search of annual reports.
http://intramural.nih.gov/search/index.tml
this website lets you search keywords of the annual reports for the different PI's at hte NIH. the annual report is an NIH researcher's general "what i'm researching on, and here are my recent papers" page. this is extemely useful to get a clue as to what a researcher might be studying and also how productive they are.
next, you should start emailing away. you can find someone's email address using this website:
http://ned.nih.gov/
as some of the people have mentioned above, its important you try for primarily PI's instead of your run-of-the-mill postdoc, cause they are the ones who ultimately decide to hire you. if you find someone you want to work for who isn't a PI, find out what lab they are in and email their superior.
finally, email LOTS of people. i sent out letters of interest to over 20 people at first, and got back responses from maybe 5 of them. persistence is the key here!
if you have any more questions, feel free to PM me!
-jojo