Weigh at the same time in the same clothes every day. Count calories consumed and burned each day. Track those on a spreadsheet with charts. Look at it every day and make small adjustments to get the averages where you want them so you lose weight slowly or maintain.

Some psych tricks I use are to have some low calorie/healthy foods that are "free", I don't count them against my calorie total. I go for a sum of calories consumed minus those burned so it encourages me to extra exercise to "earn" extra calories. (I've been known to run up and down some stairs to earn a cookie. :-) Also I figure out a minimum of junk food that satisfies my cravings and allow myself that and adjust calories around that.

For steady fitness, I set a ridiculously low minimum of exercise for every day. Then if I only do the minimums I only take a half step back for each two steps forward.

Also add some weights to any exercise if you can. If you run carry hand weights. If you do situps use weights too. It's free extra exercise.