xstitch, I did something similar for my wedding. In fact one of my guests said it looked like I'd picked a lovely bunch of flowers on the way to the church. Which, luckily, is just what I was aiming for!
I suggest, if possible, asking at the local church to see if they have flower arranging classes. When I went, although I am not a member of the congregation, I had a great time and it only cost the price of the flowers - about £8 per week.
Basically the rules (as I see it) are:
use odd numbers of flowers - they always look better
plenty of greenery. It 'grounds' the flowers and lets them glow.
Asymmetric looks better than symmetrical.
You almost always need to cut the flowers shorter than you first think.
have a practise. And then practise again.
Good luck!