Practice with sausages. I swear - I know that sounds weird or nasty or whatever - but it works. I'm living proof. OR - if the taste or whatevefr turns you off of that - a nice, long, flexible dildo will also work.
Just practice by yourself for a while and you'll be deepthroating in no time! Have fun!
In addition to practicing, try using a cream to numb the back of your throat. There are several different brands on the market, and many have a great taste to them. It weakens the gag reflex some, until you get used to having something deep in your throat. The biggest thing is to relax, if you are tense because of previous experiences it is likely to happen again. The more relaxed you are, the more you will enjoy the experience.. as well as enjoying your partner. Take it nice and slow and easy at first, before long you will be swallowing like a pro.