take a litegua bus from guatemala city, the lonely planet is really good and detailled on this. once there try either for one of the taxis that go round and round town (ask locals for directions) and are relatively safe as you´re no on your own, try one of the yellow buses if going to sto tomas or get a 'normal' taxi by phoning, otherwise sizewise walking is not a problem although you should be careful at night.
For more info on getting to livingston, check out this website: http://www.bootsnall.com/cgi-bin/gt/camericatravelguides/mar00livingston.shtml
I'll post more on the town if it gets recognized by VT as a city.
The photo is from that site.