If you can get them to post (I joined about a week and a half ago and so far the bonus miles they said I'd get within 1 to 10 days haven't shown up yet at Frontier), eMiles will give you a couple hundred miles for signing up (if you haven't signed up for them before elsewhere, at least):
https://www.e-miles.com/enroll.do;js...=frontierne1v2
You have to get periodic emails for a while, but you can select to have them sent only once a week.
If you haven't applied yet for the Frontier MasterCard, you can get one with no annual fee and 5000 bonus miles on first purchase:
http://www.frontierairlines.com/fron...name=worldcard
If you have one already, just charge a couple bucks to it once every year or two!
(This card only earns one mile for every $2 purchased, so it's not great for actually using day-to-day,
except for the bonus miles and to keep your account alive.)
Keep in mind that miles expire at the
end of the 2nd year. So you have a number of months to get this taken care of.