Veggies. This is the ultimate option for filling up on few calories (and staying nutritous!). Salads are great. I found the frozen steam in bag veggies are a good option when I'm just craving hot cooked food (some have rice, pasta, etc and include a sauce which gives it a better flavor and the entire bag is usually 150-250 cals which isn't bad). Spaghetti squash is worth trying if you're a pasta fan trying to cut calories. I find putting my pasta sauce on there is a good alternative to fill the pasta craving.

Nonfat yogurts and cottage cheese. If you can handle the non-fat greek yogurt it's a good option. Lots of protein. Light and fit is 80 cals for the container. As a bonus freezes nicely if you pop it in the freezer for about 30-60 minutes. Keeps me from eating the giant bowl of ice cream every night A nice place to mix in some fruit too.

Shirataki noodles. A pretty adequate substitute for pasta -- about 40cals for an 8oz package -- instead of 200 cals for each cup of pasta.

Sugar free jello (10 cal per serving) and puddings (25 per serving plus whatever cals for the 1/2 cup milk of your choice).

Rice cakes. Low cal and a crunch.

Soups aren't awful is you shop for the right ones. Boullion cubes made up into broth have about 5 calories each but basically no nutrition to them. A single serving size of campbells chicken soup is something like 50 calories.

A lot of the special k and baked naturals chips and crackers are very low cal in comparison to the alternatives if you want to substitute, but not greatly nutritious.

As mentioned, lots of water especially starting out. And lots and lots of veggies. Mostly just portion control and having other "less calorie dense" options to fill in the void when you want to eat without adding much to the calories for the day.