I use positive affirmations that I write in a notebook and say out loud as I write them. This helps keep me positive as well as helping to keep me motivated. If I forget a day, I start over writing again the very next day. Feels better.
Keep more than one workout partner. Whenever possible, use all of them, but when someone can't make it, you still have a partner to work out with so you won't have that as an excuse to not go. It helps all of you to stay motivated to go to have more than one person to answer to if you don't go.
Put a picture of yourself at your heaviest with the words "Just say NO" on the refrigerator and post pictures of yourself as you want to look everywhere else in the house. When you see them, take a moment to visualize yourself as the leaner you.