QUOTE(sbpiano @ Feb 21 2012, 07:47 PM)
Best thing to do is to use the lowest note of your interval (for example if you have g to F flat), then think in the scale of the lowest note...in this example, G major. Once you have worked out that it's a 7th, then in the major scale work out the 7th note (which should be F sharp to be a major 7th), therefore a minor 7th would be a half step lower (F natural) and a diminished 7th would be half a step lower again. This works for all major/minor intervals, you just need to remember that if it's a 4th, 5th or octave it can only be perfect, diminished or augmented. If your lower note is a tricky one, for example G sharp to F sharp, try removing the sharp from each to make it G to f then think in G major rather than G sharp major. You must remember to do the same thing to each note if you do this this though!
And to add a shortcut to the above (which is concise enough and helpful advice)
augmented is one bigger than major and also one bigger than perfect
diminished is one smaller than minor and also one smaller than perfect
(semitones, that is)