Get a longer cable, because the FOH will probably not want to deal with starting your deck for you. Make sure you have everything you need! Adapters, cable, batteries, etc.
It's pretty simple really. Go up to the engineer, introduce yourself and tell him you would like to record the show and would he mind giving you a feed? (here is where you would insert "I spoke with the band and they're OK with it if you are" assuming you have permission from the musicians). He may check with them, so don't lie!
If you're friendly and low-key, he probably won't have a problem. Once you're plugged in, offer to buy him a beer. If he accepts, once you bring him the beer is a good time to politely ask him to send you a signal test (if he isn't already).
If the band is OK with it, offer to make him a copy if he'd like one. Most FOHs don't care, but a lot of them say yes just to see if they'll ever actually get a copy.
If he says yes, FOLLOW THROUGH with a copy to him immediately. Get his name and address, and send it out within a day or two. Include a brief note thanking him, as this will greatly increase your future chances of recording if he is the house engineer.