Have you been to the doctor to check everything is ok? 18 months is a long time considering you both have multiple children already.
You haven't given us much to go on as far as what you've tried yet, so I'll give you some general advice (sorry if you already know/do all of this!)... Track your cycles, get to know when you ovulate (by tracking your CM, charting your temperature, using OPKs or all of the above) and time BD around it. I think the general consensus is that every second day is enough. You could also look into the SMEP plan. Basically that's having sex every other day starting on CD8, then when you get a positive OPK do it 3 days in a row, skip a day, then do it again.
Aside from that, all I can offer is the usual "look after yourself" advice.. Lose weight if you need to, eat properly, exercise, drink lots of water, don't overdo the alcohol/caffeine (make sure hubby does all of this too). There's also a few different vitamins/supplements you guys could take for CM, egg quality, sperm health etc. but I don't know much about them!
Good luck. I hope it doesn't take you much longer. xx