Firstly, you have guessed correctly, getting your speed down is all going to be about technique, but with the times you are currently swimming, knocoking 5 or 10 seconds off your 100's is very feasible as I expect you are already quite fit.
Rose is right, make sure your regular swim session is a lot more than 500m, this will ensure you're not stuffed when you get out of the pool. Working interval sets will give you more of an idea of your pace, and how much it decreases as you get tired.
Using a pull buoy is worthwhile for extra strength, and I also find that if I'm doing 20 100's it is so dull that I have to alternate pull and swim just so my brain doesn't go into spasm. Kick sets are also really useful, start off sticking the odd 50 of fc kick into your session, then make it 100m between each set, build up so that 3 or 400 m kick is ok for you, and really concentrate on your technique at least for half of each length of your kick sets, you may find that by improving your kick your body aligns better in the water making you more streamline, and using your legs becomes second nature which will impact on your pace, its surprising how many people don't bother to kick because they just forget and concentrate on using their arms to drag them through.
I agree with Danny in that you can't race with pull puoys and floats, however using them as a swim aid rather than a crutch can really assist you to concentrate on individual aspects of your stroke. Think of swimming like piano playing, with each piece you learn you break it down, practicing one hand at a time until you can put it all together without thinking (and without sounding like my neighbours son).