Almost anything outdoors and in the air will be a big help, but at a bare minimum I would recommend good quality RG-6 cable and any decent quality scanner antenna. Some folks pooh-pooh RG-6 as it's intended for cable TV use, but I've been using good quality RG-6 for scanner antennas for over 15 years and it works just fine.
Either the Antennacraft ST2 or ST3 (available from Scanner Master) would work fine. Both have been sold under a variety of brand names for years; I've used both in outdoor and indoor (attic-mount) situations for years and they're just fine for general purpose monitoring. One of these mounted outdoors and using good quality RG-6 cable would certainly improve reception.
If you're made of money and intend to only monitor the 700/800 MHz bands, go to Tessco.com and look for a suitable omnidirectional antenna like the ones on this page:
SMR Antennas, SMR Omnis | TESSCO 800 472 7373
Since the control channels are all on 700 MHz it would be best to look for 760-806 MHz coverage at least, and if you can find one that covers 760-860 MHz (unlikely) even better. The ones in the $140-200 price range should work well enough even though they cover 806-868 MHz.
If you're technically adept there are plans on the web for building your own collinear antenna.