I've been a fan, through the planning of my own wedding and and now long afterwards, of the wise words of the lovely Meg from A Practical Wedding, and I read her site every day. (she even featured some of our wedding photos!) So I hate to disagree or nitpick when I think that she has created a wonderful thing.
But I have to confess, I have one tiny baby argument with something she advocates: doing your own wedding flowers. From a practical standpoint, I agree that it may be cheaper than having a florist do them. It's fun to call your moms and aunts and sisters together to throw together the bouquets and centerpieces. But, here's the reasons why I think you should have someone else do them for you.
You've probably never done flowers before.
1) So you don't know how to care for them, or how to tell if they're nice or not. While flower care is relatively easy, they'd be able to tell you what can hold up in the heat, or for long periods of time without water.
2) Even public "wholesale" floral sale sites have a markup. It's not really too much cheaper to order your flowers that way and then having to care for them, store them, and transport them means for me it'd be worth it to hire someone and avoid the hassle altogether.
3) Do you know what looks good together? It takes experience to get the right things together, and understand that along with their texture, size, and color. A good florist will be able to balance all those things in their minds eye and suggest flowers for the look you're going for, and also alternatives.
4) Price shouldn't be a problem. A good florist should be able to fit your floral needs into your budget and give you something breathtaking.
5) Time. Due to the delicate nature of florals, they're usually done closer to the day of the wedding than many other DIY projects. Would you rather be knee deep in foliage with your family the day before your wedding, or enjoying some down time at a spa together? Depending on how big your floral dreams are, that could be time better spent doing some other last-minute task, or enjoying your celebration.
Summary: It's easier to have someone else do your flowers for you. They will have experience with seasonality, availability, flower care, and troubleshooting that you might not have. They should be aware of your budget and work with it. They will save you time, energy and stress. They have resources. Florists are neat. Thank you.