Ok. Sorry to hear. Inexperienced riders here have recommended you sand your bars, without asking you if they're knurled. Shame. The knurling is an integral part of clamping the bars firmly. Without it, that's likely what's causing your bars to slip. You could use a file or hacksaw, and lightly cut shallow diagonal grooves again (basically, roughen up the surface). On the stem, you can go a fraction deeper and cut grooves horizontally. And, when you tighten the hex nuts, put a smalll (20cm) pipe on the end of the allen key - it will give you more leverage to do them up tight. If the allen key you're using becomes rounded, you can use a saw / grinder / file - to bring it back to a sharp hex shape again, that'll keep your bolts in good condition aswell.