This is what I personally do:
1. Always keep a really good look out, as said by others be proactive with your scan. If flying on a clear day, be aware of empty field myopia. Obviously, make sure the windscreen and windows are clean of bugs etc. Have your eyes on stalks when in the vicinity of aerodromes and indeed choke points such as beacons, corridors/pinch points between CAS and distinct land marks/VRPs (perhaps try and avoid some of these if possible), keeping clear of ILS approaches and holds were also mentioned above.
2. Keep your ears open, build up a mental picture of where other traffic is based on position reports, especially in the vicinity of aerodromes. There is no guarantee that other pilots will be reporting their position accurately. However, you can make an effort to do so and also take extra care to follow aerodrome procedures correctly and position yourself correctly. When in the vicinity of an aerodrome and you cannot see a reporting/reported aircraft, state that you are not visual with it so that the other pilot does not assume you can see him/her. Also, make an effort with the RT, keep it accurate and concise using correct phraseology.
3. In busy airspace, get a traffic service from LARS if available. The radar controller acts as a second set of eyes watching over you, this is very helpful in busy airspace such as in the SE of England, or at least listen and use a listening squawk if available.
4. Keep your cockpit tidy and organised to minimise potential ‘head down’ time. Keep information and equipment you need to hand.
5. If equipped, always use the transponder and have mode C enabled.
6. If equipped, use strobes while airborne. Also, if reciprocal heading oncoming traffic is reported and you cannot see eachother because its murky/hazy switch on your landing lights to be more visible for a while. Don’t leave them on unnecessarily though.
7. Use TCAS or a PCAS device such as the Zaon XRX, MRX or PowerFLARM etc. I have an MRX and it works well. Be aware that these will only pick up transponding traffic and consider it as just a backup/secondary safety device. This is an area where things are evolving and ADS-B will yield new possibilities for GA in the future, I recall quite a detailed thread on here about this a short while back.