How can I collect back child support?

My kids’ mother hasn’t paid child support in months, and she owes about $4000 in back support. She’s promised to pay it but does nothing, and the back amount just gets larger. Is there anything I can do to force her to make the regular payments on time and to collect on the child support arrears that she owes? When I talked to an attorney awhile back he said we couldn’t garnish her wages because she’s self-employed. I’ve read online that I might be able to intercept any tax return she gets and put it toward the child support, but I’m not sure how to do that.