Buses are extremely crowded. Ideally get on at a starting point when the bus is empty so you get a seat. If not then you'll have to stand and put up with the countless sweaty armpits and swaying elbows (I'm so glad I'm tall!) I certainly wouldn't recommend it for single foreign women as it's a groper's paradise.
If you have to get on mid-journey, get on at the back entrance and the conductor will come and find you and collect the fare. If you're lucky enough to get a seat then expect numerous bags and even children to be dumped on your lap - it's the general rule that those sitting help out the people standing.
'Ladies first' is not much of a rule on a Colombo bus! But you should give up your seat for old people and women (or men) with young children. I once tried to be polite and offer my seat to a woman with shopping bags but the men just pushed her aside and jumped in my seat before her. Monks also get priority and the front two seats always carry the sign 'reserved for clergy' - even if the monks are young and healthy and the person sitting there is an 80 year old woman she'll be expected to let them have her seat.
Horrifically disfigured and disabled beggars are common on quieter buses (as on busier buses you can't move!) so you might want to keep some small change handy.
When the bus arrives the only place I've ever seen a queue is at Borella bus station. Anywhere else it's a free-for-all and you have to fight to get on. You can't bring out a sledgehammer and break people's legs but otherwise anything goes!
Finally if you're on a route where you're not sure where you're going then try hard to keep near the window or the exit. If you let yourself be pushed into the middle of the bus there's no escape and it can take ages to fight your way back through - by which time you're miles away from where you want to be!

