Always know the size of your significant other's clothes, shoes, and ring size.
Makes for surprise shopping much less of a hassle.
More Relationship Tips
Make it a point to remember peoples' birthdays.
They will never forget you and you will gain a lot of respect from them.
Before you end a relationship with someone, reset all your passwords.
If you've shared devices together, log out of all accounts as well -- Netflix, email providers, social media accounts, etc.
A marriage proposal shouldn't come as a total surprise.
Before popping the question, make sure you and your significant other have discussed the topic together so it doesn't shock them. You can't still surprise them, but just make sure you're on the same page before you do so.