Did you take apart the slide? That spring is tough to get in, I would reccomend using a couple of punches and/or screwdrivers if that is what you are trying to do. I haven't been able to do it with bare hands and have found the use of tools makes it easier. If you are referring to replacing the mainspring housing, the top goes in first and you should finesse it in - insert, small amount of pressure, if that doesn't work insert at a slightly different angle, top in first, then apply pressure up towards the top while you press in. Once you do it once it gets easier, first time is a hassle.
There is no need to take the slide apart for regular cleaning.