Like all but the purest of the pure, I occasionally wind up with an overpowering urge to use some 25.4 bars with a 26.0 stem, or find myself trying to hang a Cyclone front derailleur on a metric 531 downtube (Gitane TdF Pro). I used to regularly slice myself making and installing "Coors" shims: beer can stock. Now I've found a Better Way: PLASTIC. The plastic stock that was used for overhead projection slides ("viewgraphs," etc) is a nice thickness and often works well. Also found as report cover stock. Since it is clear, it is much less conspicuous than metal shims if you want to avoid nicking the handlebar by trimming (for example). Of course, we only do this for "riders," and we cinch things down right good, and we would never, ever, consider showing such a rank amateurish thing at Cirque, but it doesn't seem immoral and it has worked for years for me on one bike.
Of course, your mileage may vary.