Of course it's possible to get pg just having sex omce a month in the middle of your cycle, but to be honest I'd try and put a bit more effort in if it's what you both really want! Once every other day for abouit a week around days 12-19 should vastly improve your chances. Like oopsadaisy said, look it up on fertilityfriend for your approx most fertile time.
If you really don't fancy a bunk up get some ovulation sticks, they will tell you exactly when to have sex.
Remember though once PG and especially when the baby comes you will probably not be wanting sex for ages, so maybe make hay now while the sun shines and store some intimacy up with your DP for the dry months/years to come!
Tell me about it belgianbun - DH and I are ttc#1 and I just wish my libido would come back! We're lucky if we do it once a fortnight at the moment - we're both very stressed about work and money, so I know why things have gone off the boil, but I just want to feel real desire again, not be going through the motions He is gorgeous-looking and a very considerate lover so it's not like I've gone off him (thank God! - and I know how lucky I am), but I find it really hard to "get going" at the moment, iykwim...