For tanking/healing/dpsing I use macros instead of spells directly on my alts. An example is that I use the Numpad keys to pass through keystrokes to my 'team'.

Healer: Keys 4 (self target), 5 (1st DPS), 6 (2nd DPS), 7 (3rd DPS) and 8 (Tank) target various members of the group. /, *, - and + cast various heals so a combination of 5 followed by * would target the first DPS char and then cast X type of heal on them.

DPS: Keys 0, ., 1, 2, 3 are 'DPS' and utility keys. The attack keys (generally I use 0 and 1) all use macros like this:

#showtooltip Spellname This ensures that the macro shows the correct details of what spell is used
/cast [target=party1target] Spellname This casts the spell at the party leaders target - generally my tank is always set to leader which gives me the luxury of knowing that all the DPS will actually be targeting my target for once >.<

Just a rough guide and I'm sure that the Wiki will contain tonnes more ideas on how to run a trinity setup