Not a lot to say myself right now but it's worth checking out some good old pixelly tuts:
Tsugumo's (mirror)
2d Tile Art BasicsThis one (
Seiken Densetsu 3) is a pretty long read but has a tile breakdown towards the end.
A few of the Feature Chests and Commercial Critiques are goldmines, a lot have to do with sidescrollers/tiling so check them out.
Other than this, general stuff:
Decide on the art direction
Work on more than one asset at a time (to keep everything meshing, for time constraints, for bridging the skill gulf from 1st asset to last asset)
Start with the big simple shapes then draw in the small details
Use reference (you can learn as much from a great ref as you can learn from a great tutorial)
Use placeholders
Prioritize assets
etc.
These are just the basics, if you need me to elaborate on any of those give me a shout.
Looking forward to seeing some of your project, and good luck with it man.