The following may seem like pretty common-sensical rules, but so many people miss so many things on a daily basis that a simple system is perhaps warranted:
Always get schedules - they are not always accurate but they are better than nothing; and even though it won't help if a bus is late, if a bus is early, or if the driver can take an extra coffee break - they will. So they are never too far off.
Figure out how long it will take to get there (this may take a non-hurried walk to the bus-stop to time it). Add five minutes and a further five minutes and leave that long before the bus is scheduled to come. The first five minutes is to cover yourself if you are running late, and need a little extra time; and the second five minutes is in case the bus is early1.
Before you go, check to see if you have your fare/pass.
Check again - there is nothing worse than getting on the bus and finding out you forgot your wallet.