Have a wingman, or better yet, a wingwoman. Since you’re crashing the wedding you don’t have other people to vouch for your credibility. You need someone there to back you up. Before the bridesmaid is even going to toy with the thought of hooking up with you she needs to feel confident that you aren’t some creep. While wingmen are great, wingwomen are better. Girls are less trusting of men in general. If you’ve got a girl who’s a friend and up for a good time then have her be your partner in crime. Side note: a friend is not someone you sleep with. Your wingwoman is good for many things from confirming stories that you may or may not be making up about yourself to meeting the girl first and even doing a pretend introduction. There is almost nothing a wingwoman can’t do.
Always have booze on hand. This means don’t count on the wedding to supply all of the booze. While your good friends wouldn’t get married without a full bar and five kegs, you don’t know these people. Make sure there is booze in your car, in your pants and in your coat pocket. These people may not have the kind of alcohol situation that is helpful to a crasher. The kind of crutch that bridesmaids may need to let go of their inhibitions varies and you want to make sure they are able to relax, and take their jealousy of the bride and loneliness out on you, in their hotel room.