My chapter has been in that boat too - we were by far the smallest of 5 sororities. Of course, NPC rush is a different animal...
I second chideltjen's advice. Also:
Round up a bunch of area alums to help you out. They don't even have to be from your chapter. If you do this, though, BE SURE to have a good rotation system so that most or all of the active brothers get to meet each PNM. If a PNM only meets alums, who's going to vouch for him when it comes time to vote? Also, the first person each PNM talks to should be an active, if possible.
A good selling point for a smaller chapter is the opportunity to take on a leadership role immediately. Smaller chapters are always looking for committee chairs... you have to be careful to spin this the right way, though.
Another good selling point is that you get to know all the brothers well in a short amount of time. If your fraternity has 100 brothers, how many will you know
well by initiation? Or even by graduation? One of many reasons I joined my sorority is that I had the opportunity to meet and have good long convos with all 7 sisters and all my fellow PNMs during rush... when I got my bid, there was no question that I was going to accept, because I felt that I was starting to bond with each of those women (both the sisters and my pledge sisters). Again, though, you have to spin that the right way.
And the usual stuff, that goes for big chapters as well as small: Brag about your accomplishments, be they academic, philanthropic, sports, whatever. Be very very visible - wear your letters, have a table in the student center where you all meet for lunch or dinner in the days leading up to rush, etc.
Best of luck!