As part of any regular leg workout, a glute program should be included. Most of the major exercises that work your legs will also work this muscle as well, so you will not need to dedicate a major amount of time to doing them separately.
The main exercises you want to include in a glute program are squats, lunges, one legged split squats, stiff-legged deadlifts and hamstring curls. All will help to reshape the back of your legs and prevent your bum from sagging.
Keep in mind that the heavier weight you can lift the better the shaping effects will be so aim for something that will fatigue you by no more than ten reps. If you are doing fifteen or more reps for a single exercise in your glute program it definitely is time for you to increase the weight.
On the cardio side of things, there are specific forms of cardio as well that will help zero in on this area of the body moreso than others and should be included. The first one is uphill walking. Doing this on a regular basis will not only help to challenge your cardiovascular system but will also add resistance to your thighs and hamstrings. The next option is the stepmill. This is a terrific exercise for not only burning fat but also building lower body muscle.
Just keep in mind that while you must work the muscles hard, you must also give them a chance to rest if you want them to grow back stronger. Do not neglect this aspect of your program or you may just see yourself losing strength rather than seeing results. Aim for at least one day of complete rest per week and then a day in between all heavy sessions where this body part is worked.