By loyalty and perseverance, but the other party needs to do the same. A lot of ugly crud happens in a relationship, and especially once the chemical stage of love (the first 3 or so months) ends, the real love stage begins, and that stage takes a lot more sacrifice. It's not so much a "And he layed eyes upon her and at once knew they would live happily ever after" it's more of a "And he thought she was really pretty, and worked hard to make their relationship last through the good and the bad till the day he died". Good luck though mate.
IMHO, we cannot find true love as such, but what we should do is to look for a person who matches most of our principles for a partner, and then we, the couple, should be caring and forgiving. This way, we'll have one of the sweetest experience of our lives, if not the only one.