Basically the gain of a properly-designed omnidirectional vertical antenna is a function of the
length: longer antennas have more gain. (And antennas that are about the same length have
about the same gain.) Along with gain comes directivity: in this case, maximum radiation is
compressed into a small range of vertical angles to get that gain. One problem with using such
antennas in any sort of temporary or improvised mounting is that, if they aren't close to vertical,
the gain will actually be less than a dipole in some directions because maximum radiation is
above the horizon. For example, a 6dB gain omni vertical (about 20' long on 2m) if installed 10
degrees from vertical will be worse than a dipole in the directions that it tilts. This would also be
the case if it is swaying in the wind. So higher gain antennas require more attention to how
they are mounted, in addition to the mechanical issues that Steve mentions.
One possible method of mounting longer antennas that are designed to attach to the top
of a mast would be to put them on a 20' piece of pipe / mast / chain link fence top rail and
hoist them up with the rope tied below the antenna mounting point rather than to the
antenna. Among other things, that would get the antenna itself up higher in the air. You
could even extend the mast (for example, by adding sections as you pulled it up) so that the
base was on the ground and the top supported by the rope pulling it against a tree branch:
that would allow you to move the base around to get the antenna vertical.
There are designs for higher-gain homebrew vertical antennas, such as this one (for 440,
though it would work better if it used wire inside copper pipe rather than coax):http://www.rason.org/Projects/collant/collant.htm
That's basically what is inside a StationMaster if you want to build your own, and the
concept can be extended to various lengths.