Regardless of how you feel about your girlfriend's mother, it's important to get along for the sake of peaceful family gatherings, holidays and dinners. Having a good relationship with her mother may seem like a daunting task, but making it happen will result in all involved feeling happier and more at ease. Although you don't need to love her, try to build a peaceful relationship through a few simple steps; a happier girlfriend and girlfriend's mother means a happier life for you, as well.
Dress appropriately and suitable for your age. You need not always present yourself in a suit and tie, but avoid dirty workout clothes or mud-covered hunting gear. Choose a collared shirt or tuck in a casual shirt, even if you are just going over to your girlfriend's house for a simple dinner.
Treat her mother with respect by addressing her as "Miss," "Ms." or "Mrs." Say "yes ma'am" and "no ma'am," unless she specifically requests otherwise. Remember the rules you learned as a child about how to treat your own mother and use those as guidelines for how to treat your girlfriend's mother.
Watch your language -- be cognizant of the appropriateness of topics you talk about in her presence. This isn't the time to act like you're at the bar with your buddies. Avoid cursing or talking about the weekend you spent in Vegas for your best friend's bachelor party. Don't hide who you are, but don't flaunt this part of your life either.
Treat your girlfriend's mother to a special dinner or gift to show you appreciate her. Sending her flowers or chocolates on Mother's Day, her birthday or any day of the year isn't bribery, but a way of showing her you respect and acknowledge her. If you are looking for a less expensive option, a quick note expressing your appreciation for the dinner she fixed for you is also a nice gesture.