Sidescroller level set-up ideas/best practice

Just mulling around the best way to set up levels in a 2d platformer type of game.
Do you just set everything in the stage in Flash and then define which objects are hard etc? I was thinking about making a big image to put in the background and just move that behind the character, and have the walls represented by an invisible layer that just delineates what you can go through and what you can’t…
would you have repeating elements that are just specifically positioned in a level.as?
I know it can be any of the above, I’m just curious if there’s an overall sort of best way before I try all of them to find out :stuck_out_tongue: