How to improve my sprint

Hi everybody, first of all I'm glad to know you. I'm an italian master and I'd like to improve my sprint. My strokes are: Free (50 & 100), Fly (50 & 100), IM (100 & 200) I've a problem every time I've a race. I can't mantain my speed till the end of the race. For instance, 50 mt free. 35/40 mt are ok, then I go down so quickly. Same thing with 50 mt Fly. My 100 mt free are: 60/70 mt ok, then ... boom :-) I really would like to change this way to race. Do you have some suggestions to improve my stroke? Some good workout to do ? Just some information to understand what kind of swimmer I am: - I swim 3/4 times per week, 1 hour per time. - Some personal best: 50 free (27"50) ; 50 fly (30"00) ; 100 free (1'02"50). Thanks a lot in advance Giampiero P.S: I'm sorry about my english :-)