Unfortunately alot of it has to do with keeping your "situational awareness" ect ect, what I personally do is set a timer (an audible alarm) for 20 minutes before the 2 contacts meet. That gives me enough time to issue the requirement to ensure separation before 10 minutes to time of passing. However at 3 in the morning, the caffeine has worn off, you are on your 9th shift in a row and your neighbours garage band was practicing all arvo and you have had 2 hours sleep.... well, then anything is possible. The scary thing is that in procedural airspace the first alarm we receive is you guys responding to a TCAS RA.