Tips to make your game more smooth? (less lag?)

Some things you can do without upgrading your rig:
Play on smaller lots with less furnishings.
Reduce the size of Sims' inventories, both household and personal.
Reduce animated objects on lots such as fountains, animated lawn ornaments, etc.
Reduce the number of large mirrors in your house, especially multiple mirrors that cover whole walls, etc.
Play with smaller households, 1-4, versus 5-8 Sims.
Play with lower graphics settings.
That's a few off the top of my head, there may be more.