Don't try to change them into something of your ideal person. If when you meet someone and you think they would be perfect apart from a couple of things walk away they will become mountains
Listen to what they say and show some interest in their interests, you don't have to do them.
Let them have space to do things they enjoy that you are not so keen on.
Let them know you still desire them/love them etc, don't be overbearing/sleazy on the desire one.
Be honest with them, don't lead someone on when you are hoping something better will come along.
Be honest about children and accept people may change their mind.
Make an effort for them, once you are married its not good to suddenly become a slob.
Work at it, put the effort in, if they had a bad day or family etc are causing them pain give them the shoulder to cry on and your total support.