The most important thing is to make sure that the bike fits you. If it doesn't fit, you're gonna be miserable, regardless of what tri components you put on it. Assuming that the bike fits you, the first thing I'd do is slap a set of aerobars on it. You don't really need to go with a full tri setup with bull horn bars, just any aerobars clip-ons will be fine. The second, optional, thing is aero wheels.
Some people use a forward style seatpost to give them a steeper seat tube angle, but I don't really care for them. It makes the bike look weird, plus it can throw off the balance of the bike. The theory behind this set up is to give you more of a lying down position (more aero) and to work you quads more. The idea is to save your ham's for the run, but I've talked to my tri friends and they can't confirm this theory, either way.
I'm making the assumption that you already have clip-less pedals.