Tie Downs (or Guy Lines) are used for many things while camping. They can be used to brace/secure tents, hang clothes to dry, and to anchor tarps to keep your tents dry.
Match the strength of your tie down to the task at hand. Most tents are pretty stable, so a tie down used to brace a tent does not have to be of great strength unless you are expecting some very strong winds. Hanging clothes on the other hand requires a lot of strength. I usually carry a piece of nylon rope, or cotton clothes line for this purpose.
When tying down tarps I usually start by attaching the tie down to the tarp. All my tie downs have a loop tied in one end, and I thread this loop into a grommet on the tarp, then thread the other end of the tie down through the loop, and pull it tight. This is a perfectly secure attachment that will be fairly easy to remove later. I then stretch the tie down out and decide what direction I want to pull the tarp in, and weather I want to keep it level, pull it up, or pull it down. Once I know that I can choose to tie to a tree, another tarp, or a tent peg. If I want to tie to a point that the tie down will not reach I tie two tie downs together. If it is a tarp I am tying to then I just tie the line directly to a grommet on the tarp. If I am tying to a ten peg I hook one end of a bungee cord to the peg, then tie to the other end of the bungee. If it is a tree I may tie directly to it, or I may stretch a bungee around the tree and tie to both hooks, or I may use another tie down to attach one end of a bungee to the tree, then tie to the free end of the bungee. This all depends on how the rest of the tarp is attached, and whether I think there is enough stretch in the lines already or not.
I inspect my tie downs as I use them, and any that show signs of abrasion or other wear are discarded. My knots are carefully chosen for strength, ease of tying, and ease of untying. When ever there is a long loose end after tying the knots, I take the time to roll it up and secure it. Any tie down that is going to be near head height is a bright neon color to help avoid accidents, and I try to keep any tie downs that are in the way of normal travel in the camp site high enough so they can be walked under.