Become a fan of h2g2
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.
1Note that this way, if you really need it, you have ten extra minutes to protect yourself if, say, your clothes aren't dry, or the driver is very early.