Learn how sims age and how you want to play - start with one household, but check out if rotational play is for you. A rotation brings more variety, and it might also trig your stories in many ways. Understanding how sims ageing, career promotions, bills etc are affected by moving about vs staying on one lot is essential to plan a good routine.
Take your time to enjoy each addition. Getting many new packs added at once might be twice the fun, as some packs even get better when mixed. But the content might also be overwhelming, resulting in getting the feeling of not staying on top of it all. It takes a while to get familiar with new features, ie most packs offer objects with new functionality, more social interactions and some adjusted UI. Careers often bring new stuff at the higher levels, not taking the time to get there means you'll miss some of the fun.
Check out the buyable extra traits! Some might look pretty useless with the base game only, but with a few packs added, you'll see them in a new light.
Some packs might not add a lot of obvious content, you might think Seasons is just about weather. But it's not. It changes how you'll see each of your worlds and lots - it brings tons of visual variety to the game, and also challenges you to plan your events differently (who wants their garden wedding in Winter anyway?). Same with clubs system in Get Together, it can be used as a plain club system, but it's also very in handy for bringing sims together - like gathering relatives! Throwing a wedding allows your sim to only invite sims s/he already met, but with a club packed with relatives old Auntie will no longer be left out :)
Use the gallery if you don't want to build everything yourself, or create all sims. Anything you grab there can be modified by you now or later.
