what is the best WAY?

Cook for her! Or whenever you are at her house, help in the kitchen or something to show her that you can also take care of her son like she can. Not in any way competitve.

I think the best way is to act normally and to talk to hey like is your mother but attention!!! Don't call her mother! :) Try to be good with you boyfriend mother and she will understand that you are a good girl and you love his boy.And if he wants to be a normal relation between you and his mother try to explain him that he must make her mother understant thet you make him happy and he loves you :)

try to be very kind, to give her some presents or flowers... maybe you can win her heart, but to be honest i think that it is very difficult, expeccially if your boyfriend is the only son!

when you visit your boyfriend's home, always wear nice dress (as per his mothers view), and give proper attention to herself, when you are there try to help her in her household works, be polite, give her proper respect.... share good thoughts with her, you must have communication channel with her then only she can understand you, so mean while call her , at any special day ,wish her , its Xmas time call her (specially her) to say marry xmas, it will help you a lot..