can you wrap some plastic tape (like insulation tape) round the wheelbarrow handles, then push on the handle grips, so it's a really nice tight fit ?
i like that advice cufcskim
If you know a friendly electrician or have access to an electrical distributor, heat shrink tubing is excellent and will outlive the life of your barrow.
Basically, you measure the outside diameter of your wheelbarrow handle, acquire a foot or so of the right size tubing, cut it to length, then using a blow torch, shrink it on to the handles.
They make very comfortable & permanent grips!