There's two ways:
1) Using weights with the diet & cardio to protect lean mass,
Lean mass is a real metabolic driver; muscle chews up a lot of calories so it's a good idea to protect this. When you diet hard and or do a lot of cardio you lose muscle. Keep the muscle increase the fat loss rate.
Also the very act of weight training will chew up calories in a different way and make your body USE the calories you are eat in a different way meaning that nutrient partitioning is pushed in favour of lean mass. (i.e. for every 1000 cals you eat a proportion - not a lot but they add up - will end up as lean mass instead of fat mass)
Example routine for keeping fat mass without wanting to bodybuild
Tue: deadlift, lat pull downs, overhead press 4 sets of 8. 20 mins cardio after
Thurs: Squat, bench press, rows 3 sets of 12, 20 mins cardio after
2) Using weights as a form of 'cardio'
You can use weights to give you a cardio type training result but is is hard work. good terms like 'cluster training' and have a look at the article I worte on tabata.... http://www.muscletalk.co.uk/article-tabata-training.aspx
training like this isn't for the in exprerineced.
I would recommend choice 1.