I have a custom template that I created and use for every new save. It's set up with modified lots, premades and copies of all my favorite Sims from other saves as 'not-in-world' townies. All I need to do is; open that template, click on the 'create new Sim to move in' button at the lower right of the Manage Worlds screen, create my Sim(s), place them on a lot, build them a house (or not) and start playing. When I'm ready to save gameplay for that new save, I choose 'save as' and rename it something appropriate. Lather, rinse, repeat for the next new save. :)
Whenever I get a new pack, I just update the custom template for any new premades/worlds to my preferences and carry on as usual for the next new save. Using a custom template has made everything so much faster when starting a new save. I don't have to spend time updating everything before I can start playing. I love it! <3
EDIT: Forgot to add that I always create a folder for any new save and place any screenshots I've taken in it. I also create sub-folders inside that folder for the Sims/Lot tray files and for their appropriate save game files too if I'm finished with the save file or it's on hiatus to play later. This has made it a lot easier when picking/choosing which Sims I wanted to put in my custom template. Also, if I ever want to reuse a house in another save, I have them in those folders to easily drop in the current tray file.
It helps to streamline my game too by not having it cluttered up with a ton of Screenshots/Sims/Lots/Saves in game all the time. I have very little lag (if any) as a result.