(1) Code reviews. At first, just get 'em to grab a passer by to look at their code prior to check-in. If the PB cannot understand what they've done, they haven't finished the job. Later on you can upgrade to more stringent reviews, but the first thing is to get *some* reviews happening *at all*.
(2) Comments and (some) documentation. You need to lead by example. This stuff isn't optional.
(3) Unit testing. If your code base is a pig, you'll need to start somewhere and begin teasing out the "bits that think" (easier to UT) from the "bits that talk to things" (these are harder to UT and you can get away with integration testing here until you're in better shape). Unit testing is a skill anyone can learn. Sack 'em if they refuse to do it!
(4) Simplify your processes and your architecture where possible. Avoid trendy bandwagons. If the obvious thing works and is maintainable, then that is probably the best way to go.