Don't play the blame game. Constantly saying "You always do this" or "You always don't do that" will make the person feel as if you are always placing the blame on them. For example: If you are upset about something say "It hurts me that we don't communicate." instead of "You never listen!"
Take time out for yourselves! Especially if you have children! Get a sitter and do something together; even if it is only once a month.
Communication! Take the time to listen to each other! Never say something in anger that you may regret later! Walk away and calm down; then approach the situation when you are more level headed.
Don't put you children in the middle! The should be able to love each of you without fealing gulity or facing ridicule from the other parent. Just because you don't get along doesn't mean the children need to pick sides! Fight fair!
Note to guys: Remember important DATES! Keep a Calender so you don't forget! This matters to a lot of women (anniversaries; birthdays; day you first met; etc).
Don't gripe over the small things. He leaves the toilet seat up? You bite your nails? HE leaves his dirty socks in the hallway? You leave the cap off the toothpaste? You both have quirks that drive each other crazy. learn to compromise and let most of them pass. Make a list of things that you'll let slide and things that are a "Hell NO." Tell each other these quirks.
It's the little things that count! Buy her flowers unexpectedly; give him a foot rub; make her dinner; dress in something sexy; let her sleep in; etc!