This is my first instructable! Anyway, I recently moved into a new apartment and having a background in animal behavior, I wanted a bird feeder. But! No matter where I seemed to hang it from, the squirrels were able to jump to it and knock down most of the seeds or break the feeder. They would climb onto the window ledge pictured here and jump at the feeder to knock seeds onto the porch. So this is my solution:

Step 1: Monofilament Line

First, this instructable is for deterring squirrels from jumping, not necessarily for preventing any kind of climbing. It also assumes your feeder is hung from some overhanging structure. That being said...
1.) Attain monofilament/fishing line:
Tend to the thinner side as it will be less likely to deter birds as well as limiting an obstructed view of your feeder.
2.) Cut the line to the height of your structure (porch, etc.) and attach a weight of any kind on one end of each length (I used 5/16 nuts)

Step 2: Attaching Lines

I am not allowed to drill into the roof of this rental property, so i attached each line to the overhanging roof with duct tape...making sure to leave an excess of about 4 inches out the other side of the attached tape. Make sure to align the weights with eachother, as this will limit tangles. My weights were about 2 feet off the floor.

REMEMBER: It is worth stating you should not put these anywhere a bird could become entangled. Therefore, these should only be used on one, maybe two sides of your feeder. Use your own discretion.

Step 3: Reinforcing Lines

If you are using tape, pull the excess line left over taught in the opposite direction of the way it was secured and place larger pieces of tape over the original in groups of two or three at a time. This will give the most amount of friction to the system in order to keep the lines secured in their attachments. (The picture is just of the line before it was doubled back on itself)

Step 4: Finishing the Project

Make sure to clip the excess filament from the weights. We want the least amount of surface area that could move the weights and thus tangle the lines.

For an extra stabilizer, you could string a filament through the nuts (if that is what you used), scotch tape them together as shown in the picture, or anything you can think of. Just be careful it cannot be used as a stepping stone for squirrels. Even if they cannot get anywhere by using it, their weight could endanger the system.

I scotch taped a few together at a time to allow me to check squirrel activity when I was not around and to have a stabilizer that could break away if too much weight was applied (i.e. squirrel)