To perform Quickenings more easily, hold R2 when Triangle, Square, and X are not displayed, then quickly tap Triangle + Square + X. This will assist in capturing more repeat attacks, multiplying the damage you inflict on enemies. -From: Onision

Use less MP for Quickenings

To perform Quickenings without using much MP, remove all but one member from the active party list. Perform the Quickening, then once it expires and all MP is gone for that character, add another character with full MP to your party (two total characters now). Initiate their Quickening, then continue rotating in and out one character with full MP. This will enable you to use character Quickenings over and over again without draining everyone's MP at once. You can use other character's Quickenings regardless of whether they have MP or not as long as the one who initiated the act has MP. If you do this with all characters having MP, they will all lose their MP Max, which cuts down attack value by three times.