I teach my sixth graders that you can only add/subtract things that are the same and you multiply/divide things that are different. Have them look for the labelsof the numbers to see if they are the same or different.
I also teach the (a) five step approach to problemsolving and I make them write down each step -- question, data, plan, do it (which includes an estimate), check. They also must use at least 1/4 of a sheet of paper for each problem.
This might help -- it takes me to about htis time of the year to convince the kids this is how we do it but it does seem to work.