If you have your documents in order and are willing to answer a few questions about the aim of your visit, entering Bolivia should be a breeze. If crossing at a small border post, you may be asked to pay an ‘exit fee.’ In most cases, such fees are strictly unofficial, but it's easier just to pay them anyway.
Note that more remote Bolivian border opening times can be unreliable at best and it is worth checking with a migración (immigration) office in the nearest major town. If you plan to cross the border outside the stated hours, or at a point where there is no border post, you can usually obtain an exit/entry stamp from the nearest migración office on departure or arrival.
US citizens need a visa to visit Bolivia. Citizens of Australia, Canada, New Zealand and most European countries do not need a visa.
Border agents may or may not request a yellow-fever vaccination certificate, but there are occasional checkpoints heading into the lowlands, where you will need to produce a certificate. Some neighboring countries, including Brazil, require anyone entering from Bolivia to have proof of a yellow-fever vaccination. If necessary, a jab can often be administered at the border but it is preferable to take care of this at home.