Cheap Plane Tix Tips?

So I did about an hours worth of researching last night on different plane ticket sites and I ended up finding a 2 week trip from Denver, to STL, to Dallas, and back to Denver for only $234 WITH TAX!I go on tonight to buy the tickets and everything has gone up $50-$80 then what it used to be, what the heck?! Is this normal? (I am not very plane ticket savvy, never bought tickets this far in advance before!)Should I wait till tomorrow or within the week to buy tix? I am scared for them to rise even more. Anyone have tips on how I can save money? (PS - I am hoping to do my vacation between May 16-31st) Thanks!