Mass-effect – access the developer’s console in Mass Effect 3

Well, hopefully you'll find a better answer, but one workaround is to simply back up some of your older saves to another directory.

On Windows 7, my saves are stored in

C:\Users\MyUserName\Documents\BioWare\Mass Effect 2\Save\John_12_Soldier_260110

That last bit differs on a character-by-character basis. You could grab the .pcsav files from there and move them to a backup directory. Now you've got some archived saves and free slots. If you want to restore them later, you just drop them back in (though you probably need to make sure you aren't bringing the total over 50 if you do).

Firstly for an initial point of no return - as soon as you get access to the Sirta vendor in the Hospital ward buy a/the present/s, completing missions removes them quickly before you know their value.

Secondly before beginning Priority: Tuchanka you should finish off as many sidequests as you can, since completing it renders some sidequests as failed. Don't panic about having some remaining in your journal however, a few don't have their locations unlocked for retrieving quest items until later.

Thirdly Priority: Cerberus Headquarters is the point of no return for the rest of the sidequests. You can still use the onboard vendors on the normandy to purchase goods, but because of the price markup on the ship it's better to visit them directly for whatever you need before commencing this mission as well.