Nail knot is simple with the right tool. I prefer a large double-ended needle.

If you have a loop already, look up the perfection loop. It's easy and fast.

 

I use needle nose, or forceps to tie my nail knots because there usually what I have on me.. but when my leader is getting short, I just leave about a half foot of leader on my line, and tie a prefection loop in the end of that.. that way you can just loop to loop leaders on no problem.. and the big ugly loop to loop knot is high enough in your line to not spook fish or affect casting ability..

 

Thanks guys, that was a good idea! A perfection loop I can do...just not perfectly!!

 

Nail knot or bust

I have to agree with juro. Nail knots are the way to go if you have the right tool. Sooo much faster, and a better finished product that the loops. For reference, I am referring to a tool like this :