What do you do when the bus bike rack is full? Sometimes I take commute via bike, other times on the bus, but I rarely plan on doing both because I worry about the possibility of waiting for a bus only to find that its bike rack is full and having to wait for the next bus, which may also have a full bike rack.

What do you do in this case? I asked a bus driver today on the way home and I couldn't really understand what he was saying, but he said something about a bike rack in the back and I think he was saying that were he not driving the rapid bus and were his schedule different, he would put any extra bikes in the back. Not sure though, he could have said something completely different.
There's a couple options you can take in this case. 


Ask the bus driver really nicely if you can get on the bus with your bike. I've done this a few times, not because the rack was full, but because there was no rack where there should have been one.  If the bus is quite empty, you can often use the wheelchair accessible seats as a place to put your bike.  Don't be surprised if the driver doesn't let you.  There is no way you can do this during rush hour unless your route isn't used very much.
Go on the train/subway. Depending on the rules in your locality, it's often allowed to take your bike on the train.  Even if you have to go out of your way to get to the train, it can be better then waiting for the bus only to find out the racks are empty. Again, maybe not possible during rush hour.
Cycle to a location that's before where the majority of people board the bus. Take the following scenario.  Assuming you are downtown, and want to travel home, Travel in the reverse direction along your bus route to a location before the bus gets downtown, or to the first stop if the bus starts downtown. This will give you a better chance of getting to use the rack.
If you work at a place that lets you have flexible work hours, adjust your schedule so it isn't during rush hour.
Look for a different bus route.  You might have to cycle a bit further to get to the other route, and take a slower bus  (more stops, residential roads, non-direct route) .  
Plan a completely different route, and avoid the bus altogether. For a short time I had a 25 km bike ride to work. I would take the bus in the morning as I could get on at the beginning of the route and almost always got a spot on the rack. However in the evening, after a week or so of waiting upwards of half an hour and sometimes giving up at getting a rack spot, I decided it would just be quicker to bike all the way home. Got in really good shape that summer.