A:
I had this problem when I first started using braid in the 90's. To make sure the line doesn't slip use about three feet of monofilament that has roughly the same diameter as the braid. Tie the mono to the braid using a blood knot, or other knot you can google online. A very simple knot that has decent strength is to double the line and tie an overhand knot. The main line is that if a fish has taken you down to your last five feet you've lost the battle already. That being said, with braided line don't let the fish clean you out, put additional pressure on the spool or tighten the drag before you're down to the bottom of your spool.
Ok, back to the answer: once you have a strong knot connecting the braid to the mono starter use this simple knot to anchor the mon to the spool arbor. Tie two overhand knots to the very end of the mono then tie an overhand knot so that the end of the line stops at the knot you just tied. I double the line around the arbor twice then I tie the overhand knot and move the line to the side until it is snug against the anchor knot.
That should do it, use youtube or google for a better knot description, BUT the mono is a must.
tight lines