On 2m and 70cm, the antenna and its height do more for your signal than almost anything else. This guide covers the basics, plus the one antenna-related condition the class licence imposes. Much of it is general engineering practice rather than regulation.
1Start with a better antenna
The short antenna supplied with a handheld is a compromise. A longer handheld antenna, or a mobile or base antenna mounted as high and clear as you can manage, usually makes a bigger difference than extra power, and it helps on receive too.
2Rough element lengths
A quarter wavelength in free space is about 49 cm at 146 MHz and about 17 cm at 439 MHz. That's straight arithmetic from the speed of light divided by frequency; real elements are cut slightly shorter, so trim gradually and check with an SWR meter or antenna analyser.
3Polarisation
By convention, FM and repeater work uses vertical polarisation, while SSB and weak-signal work uses horizontal. Matching the polarisation of the stations you want to work makes a real difference.
4Electromagnetic energy (EME)
Schedule 1 of the class licence sets "Conditions about electromagnetic energy". Compliance is a licence condition, not a suggestion. Before running high power into a high-gain antenna, check EME exposure for people near the antenna, including your neighbours.
5Remember who else is on 70cm
Amateurs are secondary on 70cm, with Radiolocation (Defence) primary (WIA band plan). Good antenna and station practice includes making sure you don't cause interference.