| Absolute easiest and cleanest way to do this:
Choose a solvent rated to remove common adhesives. I use "Goof Off" made by Lilly Industries. Needs to be used in a well ventilated area though. Using disposable latex gloves is a good idea also.
Here's the steps:
Lay down a piece of corrugated cardboard, a little longer than the door sills porous side up (not the glossy side). This will collect any spillage.
Then lay down cloth strips about 6 inches wide by as long as the sill plates, preferably something like an old cotton bed sheet. Paper towels can also be used as well.
Lay the sill plate on the cloth adhesive side up, score the "gunk" with a utility knife.
Fold over the cloth strips, then saturate the cloth with the solvent. Do both at once in the same cloth if you'd like, just a little messier. Just lay shiny side to shiny side. Wrap with a plastic wrap if you'd like, but not really needed.
Walk away for 5 minutes or so.
Unwrap and scrape the goo off with a flat plastic blade. I use the faux credit cards they send in the mail. Then clean up fully with a clean cloth saturated with the same solvent.
For the sills themselves it's a lot easier. Carefully pour a small amount of the solvent in the "well", the indentation where the plate sits. You don't need much, just be sure to spread it evenly saturation the entire area. For this wait only about 2 minutes or so, it comes up real easily. Again, clean up with a clean rag with solvent.
Sounds like a lot of steps, but it's really not. The best part is leaving it soaking in the juice allows it to just slide off.  | 
