Antennae changeover relays
On the common frequencies used for ATV (146 MHz above) the antennae changeover relay is a critical component and must be low loss so not to adversely affect both transmit and receive performance.
146 and 70cms
A good paper to read is Ian White's DG8 pre-amp article - http://www.ifwtech.co.uk/g3sek/vhfdx/dg8-preamp-v7.pdf - it's designed around 145 MHz but cover a lot of good practical engineering practice for pre-amps etc and does talk a little bit about the need for good relays.
For 70 cm you can use reasonably cheap BNC relays and the Cx540 N type relay which are often available surplus.
On the input you can use something like the RS951 relay and you can pick them up at rallys for a couple of quid!
1.3GHz (23cms) and above
For the microwave bands (1.3Ghz and above) you need microwave relays. These come in many types but the most common are TNC, N or SMA. Good N-type relays (not the CX520s) tend to be expensive. SMA relays are much more common for about £10 and up at a rally or microwave round table. It helps to know what you are looking for, RLC, Radiall, Teledyne etc.
Typical SMA relays which are good up to 10GHz are shown below:
Whilst 12v operation is desirable, most surplus relays are 28v operation. This can easily be overcome by using a switching regulator from ebay as shown in the picture (search for step up buck regulator) to boost the 12volts on transmit to 24v.
Some relays require driving in to both positions - this can easily be achieved by using another DC changeover relay which is driven by the PTT line and switches the c/o relay DC supply to appropriate contacts.
Tx / Rx sequencing
It always pays to use sequenced switching on tx / rx changeover - this ensures you don't try to switch the expensive antennae change over relay when the transmitter is still on. It also protects your pre-amp by switching it off before you transmit. The desired sequence is as follows:
To go to Tx:
1) Turn off receiver pre-amp
2) Switch ant c/o relay to transmit
3) Apply power to PA
To go to rx
a) Turn off Tx
b) Switch ant c/o relay to receive
c) Apply power to pre-amp.
Many circuits and pre-built designs are available including this simple single chip design by Ian Waters G3KKD published in CQ-TV 125.
The PIC controlled sequencer at shortwaveradio.co.uk called TROPIC may well be a good choice. The kit is listed at £11.50.
Transfer or 4 port relays
A lot of surplus microwave relays are transfer relays - these can easily be recognised by having 4 connectors or ports. These are entirely suitable for use as 3 port change over relays and can actually be used to provide additional protection to your pre-amp by putting it in to a 50 ohm load on transmit.