Lots of brides look to cut corners on their wedding by making some of the details or hiring friends and relatives as their wedding vendors. I've written before about hiring family and friends as your vendors but I spoke today with a friend who mentioned having her aunt do her wedding dress alterations.
I highly suggest you have a seamstress who has a lot of experience with working on wedding dresses to do your alterations. Hemming a wedding dress isn't like hemming a pair of slacks, and it's not so easy to manipulate a delicate fabric, embroidery and embellishments.
There a lot of ways to DIY your wedding in a "safe" way: favors, invitations, calligraphy are a few that come to mind. But it's very important that your wedding dress be fitted to you by a professional so that it fits you like a glove and it looks the way you want it to look on your wedding day. Remember that your wedding dress will be the most important fashion purchase you ever make and you'll be photographed in it all day and night so ensure that you look and feel beautiful in it by spending the money on a professional.