Use a razorblade with one of those plastic holders and work the edges. This process does take patience so good music and a fine glass of wine should keep you in the proper mood. When the decal is gone then orange cleaner and/or goo gone should remove any residual adhesive. If the finish is brushed then one of the grades of scotchbrite should match well. When done take a rag and lemonpledge and wipe the bike...should look good.
If the decals are the plastic/metallic type this should work well.