1) Day Currency
To carry passengers, you need to log 3 takeoffs and landings within the preceding 90 days. Remember, this to applies to each aircraft category, class and type. If you get current in an Cessna 172 (ASEL), you aren't current in an Piper Seminole (AMEL) - you must also get current in that aircraft class. This also applies to night currency!

2) Night Currency
FAR 61.57 requires that in the preceding 90 days, you've logged 3 takeoffs and landings to a full stop from the time between one hour after sunset to one hour before sunrise, if you want to carry passengers at night.

3) Instrument Currency
If you have your instrument rating, you need to fly and log at least 6 instrument approaches, as well as holding, tracking and intercepting with the 6 calendar months preceding the month of your flight.

4) Medical
FAR 61.23 outlines medical durations. Any medical certificate is valid for 60 months unless you're over 40 years old, in which case it's valid for 24 months. The difference in valid times, according to 61.23, depends on what sort of operation you use the medical for. As an ATP, for instance, a Class 1 may only be valid for 6 or 12 months. But no matter what, for either 60 or 24 months, you will have the privileges specified under a 3rd class medical regardless of which class you hold.

5) CFI Certificate
If you hold a CFI certificate, it's valid for 24 calendar months. FAR 61.197 outlines how you can renew it.

6) Flight Review
FAR 61.56 requires each pilot certificate holder to complete a flight review every 24 calendar months. Make sure you read the regulation, because there are some exceptions to the rule.

7) Part 141 Standardization Flight
If you flight instruct at a Part 141 school, you need to complete an annual standardization flight for each aircraft you instruct in.

8) Temporary Certificates
After completing your practical test, you'll get a temporary certificate. It's valid for 120 days, so if you haven't received your permanent certificate within this time frame, be sure to contact the FAA before you fly.