Something else I have found in my capping career, Kalah, and worth mentioning here. Q's (as well as other cap ships) often carry with them a supply of heavy missiles - Firestorms, etc. Unless you get them to exhaust their supply of them, you may find that it launches one at you when you are reducing the shields and, if the shields are already reduced enough, this will mean a premature and unsuccessful end to your capture. *ka-bloooey*
I usually park my HCF a good way off and harass the Q with a fighter (normally my overtuned LX) until it launches a Firestorm. If the Q's shields are fresh, you can proceed to detonate the missile from distance and help your subsequent cap or, if the shields are already depleted, wait until the missile is some distance from the Q and then fire on it. If you are a halfway decent pilot you can get close to the Q to scan it and determine what missiles, and how many, it has on board.
Once the missiles are depleted, bring in your HCF, transport over and begin the cap.
Just for kicks, I also like to take on capping Q's with an untuned HCF - makes the battle more exciting!!!