You may not think that you will have much to do but think again. As groomsmen you are probably close friends or relatives of the groom and therefore must take on the responsibilities of helping him to prepare for the big day (No that does not mean scare him with horror stories of married life). Take your role as groomsmen serious and help the groom the best that you can.
Not forgeting all those single bridesmaids that you'll impress by showing how responsible you can be.
Like the bridesmaids, you'll have to pay for your wedding gear. Of cause you'll be able to rent your tux which is a lot cheaper than the bridesmaids dresses. Be sure to get fitted well in advance of the big day and put a deposit down to ensure that it will be prepared for the day, after all you don't want to be walking down the aisle with a miss matched suit or even worse trousers that come to your knees
Be sure to attend all pre-wedding festivities -- engagement party, stag party, (no doubt you won't forget this one) rehearsal dinner, etc..
One of your roles is to help the best man prepare the stag night. This is your chance to book that stripper that you always wanted for your eighteenth but no one got you one.
Help the best man and bridesmaids to decorate the get-a-way car (it sounds like a bank job doesn't it?)
You will of cause meet the bridesmaids up front before the wedding which will give you the perfect chance to pick and choose
You will need to brush up on the sites for the ceremony and reception so you can help guests out with akward questions such as, "where's the toilet? and the like.
Purchase a wedding present from you to both the bride and groom. Or alternatively you and the rest of the groomesmen can chip in and give them a spectacular gift.
At the reception it is polite to dance with the bridesmaids, so if you don't know how to dance now is the time to brush up on your steps.