Mitral stenosis is actually an opening 'snap' in diastole (obviously), with a decrescendo rush of blood into the LV.
Aortic stenosis is crescendo-decrescendo, because of the buidup in pressure that has to occur to move blood through the narrowed valve. The valve still semi-opens when the pressure between LV and aorta equalizes, but not enough to move blood and therefore the pressure has to increase after the valve has opened.
Mitral regurgitation is obviously systolic and the best thing to remember with the regurg murmurs (tricuspid and aortic also) is that there is no discernible delay between the start of systole (tricuspid/mitral) or diastole (aortic/pulmonary) and the actual murmur. The valves never achieve patency and close all the way, so there is no opening sound or delay in the 'swooshing' murmur of blood rushing back through the valve. This is what they mean by a pansystolic/pandiastolic murmur.
Mitral valve prolapse is a 'click' then murmur during systole, as the valve leaflet gets pulled back by the blood flow from LV.
I would remember things like this for USMLE:
a) effect of Valsalva/carotid massage on a murmur.
b) effect of increasing/decreasing preload on a murmur
c) radiation sites of various murmurs (i.r. apex, axilla, etc.)
d) clinical findings associated with murmurs (most notably aortic regurg. and Marfans with MVP)
Hope this helps.