There is a lot of debate over leaving the liner in when shaping the beret- I've done both, and tend to leave the liner in now.
Best way I've found to shape a beret is start when you have an entire evening to devote to the shaping. Start by running hot water straight from the tap into the beret. Get it as hot as you can (ie no cold). Let run a minute or two, and work the water into the beret. Switch over to cold water, and work the cold water into the beret. Do this several times (hot then cold). Finish off with a good cold rinse.
Place the beret on your head, and looking in the mirror get the badge centred and leather rim where you want it. Smooth the beret over your head, starting from the left side, and pulling over to the right. Once it's all smooth on the top and sides, you can then start working the "flap" up under itself. Push the material up and under the flap until the bottom of the flap is even with the leather brim. Squash flat against your head. Carefully take the beret off, and ensure that the liner is not part of the flap, but just sits there on your head. Replace, and keep adjusting the beret so that it looks the way you want it.
Nows the time to watch some tube, whatever, until the beret dries. Every now and again hit the mirror and make sure that the beret isn't drying out of shape.
When you finally go to ground, take the beret off very carefully and leave it over a lamp shade or something to help it keep it's shape.
You may well have to repeat the whole process if you don't like the results, but so far (knock on wood) I haven't had to.
The beret tends to get just a little individual- units wear their's just a little differently than others, and some folk withing the unit will slide a little one way or the other. Make sure that you know what you're looking for before you start!
My two cents worth- a tight beret, with very little flap, is the way to go.
Good luck!
Garry