Jeroen Moens posted a, very well written, article on paying technical debt in a legacy codebase.
How can you get a legacy codebase under control and bring it to a new level of maturity? This post summarises my advice and lessons learned from years of working on a large legacy web application.
On a sidenote: as an Artisan I always like to sneak in a little Laravel in legacy projects. There are a lot of Illuminate components take can be used independently outside Laravel. If you want to do this too take a look at Matt Stauffer's Torch repo on GitHub.