Have you tried increasing the friction in the lever? You don't say which model Shimano bar-con you have, but if it's a friction shifter there should be an adjustment to increase the friction in the lever. On the L-600 "Fingertip Control" it is the slotted screw on the lever:



Tighten it just enough to hold the lever in place after you shift; too tight and it'll be hard to shift at all.