About 3 years ago, having bought my house and living here for a couple of years and figuring that I wasn't going anywhere for a while I decided to set up my first semi-permanent antenna (are any antennas actually "permanent?")  Up until this point, I had dealt with water intrusion and other environmental issues by simply bringing the antenna and coax inside when not in use... so, looking to waterproof my connections I turned to what I believed to be the standard in coax waterproofing - coax seal.

I won't even dignify it by posting a link, but everyone knows what it is - it looks like something that was dug out of a sewer lift station...  and it WILL cause you problems one day - not the day you put it on, but the week, month or year down the road where you have to take the shit off to undo the connection.   DO NOT USE THIS GARBAGE.

I actually discovered that it sucked about a week after using it when I went to undo one of my connections - and despite my best efforts to remove it at the time, the reasons why I hate it linger to this day.  My preferred product now: silicone tape - about the same cost of coax crap, available at a home depot near you.  Also sold at parts stores (for a little bit more.)  Goes on and comes off easily, far more durable and sucks 100% less.  For an extra tight seal - before using the silicone tape, lightly coat the coax on both sides of the splice with a little clear silicone RTV.  This will make sure the coax seal "sticks" to the cable and you get a 100% waterproof assembly.

All of the tower crews I have been around use a "courtesy wrap"  of black tape first.  Makes removal easier later.

I have a roll or unvulcanized rubber that also wotks well

The coax seal that has tar in it is awful

The silicone tape is nice.

Clear spray paint also helps

I do this with the silicone tape as well to soften any sharp edges.

I haven't gotten around to it yet but the next outdoor coax installation I do I am going to try a spray can of plasti-dip. That stuff peels right off when you want it to.

I haven't tried it, but I've been told that battery terminal spray protectant works well also.

This stuff?  I used it in a few places during my tower project.  Pain in the ass to work with though.

no it's a tape, Radio shack used to sell some (not sure id it was tar or not but seemed to be)

I like Coax-Seal a lot.  It seals really well and comes off fine for me, even many years down the road, but you have to use the right technique.  It's kind of like bubblegum.  Just start pulling it off the connector and then keep rolling it into a ball, and use the ball to pull up and remove the rest.  Doesn't take long and you're left with a nice clean shiny connector!

I also like Coax-Seal. Wrap a layer of electrical tape around connector first but here's the catch, turn the sticky side of the tape outward after the first turn. 10 seconds and a razor knife removes it with ease. Another form of "coax-seal" is butyl tape used for sealing old school windshields.