Hi Ildiko:
I usually bring some things to class that the kids can compare (different size pencils, boxes, etc.) I show them a short pencil for example and ask them if it is short or long. They say that it is short. Then I show them a shorter pencil and ask them if it is shorter than the first pencil. I do this with a number of things.
I also do a class survey and they have to compare two students and tell me which one is older, taller, younger, funnier, etc. I make a list of the students names and they have to go around the room asking questions and then answering using the comparative.
Example: Suzy/Alice old/young
1. How old is Suzy? 15 years old
2. How old is Alice? 13 years old
Suzy is older than Alice.
Alice is younger than Suzy.
For irregular forms, I usually bring in food - cookies, fruit, candy and we use good, better, best when tasting.
Good luck!