I've also heard a number of people suggest using the Specialist skill system, from Lamentation of the Flame Princess, instead of the percentages as apparently its more reliable, or something, and not super stretched out.There are several options to deal with this. One is to just use the B/X progression chart instead. Then what does the thief do after level 14? That's down to you.
Grim Reaper's masterpiece of RC errata (found here: Dungeons & Dragons Rules Cyclopedia Errata and Companion Document Download Page) also contains a suggestion. Allow the thief to attempt a skill at a level above current per dex bonus mod. (so a +2 dex bonus would mean a 3rd level thief could attempt to open locks as a 5th level thief). He also provides a complete homebrew rework for taste as well.