I've had many breeds of chicken and I've never managed to 'encourage' any of them to become broody. With many breeds of bird it has just been bred out of them (broody birds go off lay, and in these commercial times they are either wanted for meat or eggs).
If you want a broody type hen, the best choice is to find yourself a Silkie hen. Renowned for wanting to be mama's, and they make good mums too.
If you want to try to encourage the hens you have, you can buy porcelain eggs which you leave in their nest. The idea is they see the eggs in there all the time and instinct takes over - but like I say, they have never worked with any of my girls. Ever.
Good luck.
PS: Forgot to say, if you tell us what breed you have, we will be able to chuck our two'penneth in as to how likely we think they are to become broody.