After a failed attempt to throw on an old pair of tires I had rotated off my road bike (too many cuts in them and I kept getting flats on my commute), I went ahead and invested in a set of Conti Gatorskins - 25 wide. Haven't had ANY flat issues since. Run them somewhere between 90-105 psi...

Bike - $50
Tires - $70
Rack/bag - $80

Gotta love how some of my accessories are more than what I paid for the bike!