InSideOut,

few questions for you...
1. What years are you planning to collect or clipping right now?
2. are you collecting USA, world wide, or just a specific type or year?
3. how do you want to display them, stock pages, printed albums.
what years are you clipping, like a few said pre 1940 should stay on cover (if your space is permitting) or be sure someone famous or of notability didn't send the letter or receive the letter past or present as it may thus be more valuable then the stamp.
that being said, I generally float off all modern stamps if they dont come off by themselves don't force them off the paper some of the newer stamps have a non water salable adhesive. (many topics on this forum about this.) however you can always clip close and leave the backing on for now.
I'm very wary of photo albums, and dont use them like others have said there are some you can use and more you cannot.
if you must clip then leaving the post marks are good.
is there anything else, YES...
remember this is a hobby for fun dont do it one way because someone said so! do it the way you will enjoy, display and clip to your hearts content make your own pages or use store bought this stuff is about having fun and enjoying the collection you create and not being a slave to it.
ALSO, keep in mind you can make this hobby as cheap as you want it or spend thousands of dollars in the end it can be just as much fun either way! there is no need to throw money at a collection it will take root itself. look at ROD222 he has a minimalist budget but is one of the top goto guys on this board for information on just about any stamp.
remember to ask any question here, its what keeps this place alive and gives me something to read on my days off :)