I know it sounds a little dirty, but jeans shouldn't be washed at the same frequency as your other pants.
They're not made to withstand so much washing, which is why you might have found that your jeans tend to stretch and sag quickly and fall apart. The quality of jeans today have a higher elastic quality in them than the classic stiff denim we used to have to wear pre-90's. Original denim was stiff, took a long time to wear in and lasted upwards of 5 years. Todays denim is designed to give you about 1 year of wear if washed regularly, and 3-5 years if washed properly.
So how do you care for your jeans if you're not always putting them in the washing machine?
Steaming your jeans as opposed to washing them in a machine will nourish the fibres of your jeans and make them last longer and retain their shape. It also helps to prevent your jeans from losing too much colour and it gets rid of any odours they may have contracted from regular wear. The only time you should ever put your jeans in a washing machine should be when you have a stain on them, or if they haven't been washed for about 6 months. In between those times, a steamer (mine is Tefal and I picked it up from David Jones) will work wonderfully.
If you have coloured or dark jeans, they will fade much faster in the machine than blue jeans. Add a tablespoon of salt to the wash to help retain the dye and dry all of your jeans in the shade. As so many jeans have a high elastane quality, the sun will eat through the elastic and make them loose. Drying them in the shade means they're likely to last longer.