This may be completely wrong (worked great for me so far), but I throw an overhand knot in my leader (40lb) before tying it onto my main line (15lb). Works like a charm. Never had it break or slip on me. Usually only throwing 1-4oz sinkers with bottom rigs though. Haven't tried it on anything heavier than that.I wrap my doubled 17# mainline 6 times around my 50 # floro leader then pass the leader through the loop on the mainline, pull it tight and the leader slips out just before it gets tight.
What the heck am I doing wrong?