1) Pay for the travel. Don't just say you will, but actually provide the return tickets. Assuming you can get refundable ones for a sensible price, in case the visas are rejected again. Slightly risky, especially if you can't get a refund, but will prove their intent to go home again, and their ability to get home. If they're staying in a hotel, make an advance hotel booking if possible
2) Send them some money. If the money is in their bank account, they have the means to provide for themselves while in the UK. There's nothing to stop you paying for anything while they're here, and then they can send you the money back again. The point is that they're not relying on your credit card, they have the funds in case you fell out or (god forbid) worse. Risky unless you trust them, but if they're close enough family and you can afford to, then it will help. Of course, they could just use the money to fund the trip, rather than you paying for things while they're here.
3) Book tourist things - theme parks, tower of London tour etc. If they've got a clear tourist itinerary, it makes more sense
This way they can present evidence of their return travel already being paid, them having the funds to provide for themselves. If they've got hotels running up to the day of their return flight and events in that two week (or whatever) window, it's a lot easier for the visa-stamper to look at it and say "They're coming over for x time, they've got a hotel for the same time, they've got a holiday plan and the funds to provide for it... they're clearly coming for a holiday" #stamp#
Open ended applications are what give the visa agencies the jitters - give them a nice clear "this is what will happen, most of it is paid for" and they'll be much happier.
And if one of your family members has a new job/full time job/education course to return to, or some family event back home (wedding, christening?) to mention, that could help.