I was talking with a friend about his new key-less flute and he was lamenting that he would have to find songs in the same key as the flute to play. So I told him about a trick that works great (for some people) to play a song in a different key that I learned from my dad, a jazz guitarist.
Say your flute is in C but the piece you want to play is in A flat major. First think about the scale the flute plays in terms of number; C-1, D-2, E-3 and so on. Then number the notes in the piece by were they fall in the A flat major scale; A flat-1, B flat-2, C-3 and so on. Then match the numbers up. Works for minor scales too. Our imagined flute in C major would be able to play in A minor easiest so; A-1, B-2, C-3. If the piece is in E minor; E-1, F#-2, G-3 and off you go.
Some people have trouble thinking in terms of numbers but some people find that in terms of music, its less like MATH and easier than remembering to play up a 5th or down a 3rd. And if it works, use it!