Hello! Been doing Slim Fast since November (with a significant break for Christmas), so not as expert as some but not a total beginner. I'm doing it quite slowly compared to others -- had lost a stone but put a bit back on. Hopefully by the weekend I'll be back down to over a stone though.
Anyway, here are two things that have helped me.
One is waiting until as late as possible in the day to have my first shake (because I'm less hungry in the morning, and it then reduces my urge to snack in the afternoon).
The other is air-popped popcorn with salt. I LOVE popcorn, and this is a relatively low calorie treat. So I use up some snack allowance in the morning quite often with some popcorn and coffee. It's something I love eating anyway, so it makes me feel as if I'm not really doing a diet (even though eating popcorn and coffee every morning clearly isn't the actions of a normal, non-dieting person).
Re: soup. Never tried the Slim Fast soups, and although I know that the Slim Fast food is designed to make sure you get a fair whack of nutrients, there are many other low calorie soups that are full of vegetables and protein and so on, so don't let it put you off.
Good luck.