Re: Fly size and castability
There's an often touted formula for matching tippet to fly size that I think goes like this. Hook size divided by 3 = optimal x of tippet. Example: size 18 fly, 6x tippet. I don't think of it in quite such a formulaic way, but rather just use what seems to be about right for a given species.
On the poppers, if you're using 4x (4-5lb) then bumping up to 2x (8lb or so) should help. 10 or 12 lb would help even more. (in case you were wondering _x refers to tippet diameter, lb rating to strength)
Just thought of another thing that might help- make sure your leader is straight. You can straighten a leader by either stretching it or by pulling it through tightly clasped fingers. (you'll have to use some "touch" on this. if your tight grip is making the mono too hot to hold, that's too much. that much heat will weaken it as well)