First, do not use any heat to dry the stain, but hang the trousers up to dry completely. As long as the cotton fibers are damp, their capillaries are full of water. Grease and water do not mix, so by keeping the stain damp, you are trapping the grease.
When the stain is completely dry, saturate it with a hand dishwashing detergent such as "Dawn". With your fingers, not your nails, work the detergent thoroughly through the stain. Wash the trousers in cool, not warm water, into which you have dissolved more detergent. Hang to dry again. Repeat if necessary until the grease stain is gone. This may take a period of a few days.
If you are ready to give up, one last time before you pitch out the trousers, spray the stain with ordinary hair spray until it is saturated, then wash in water with detergent once more. The alcohol in the hair spray helps dissolve the grease. Hair spray works well on ball-point ink, too, which is another type of grease stain.
Many people put grease-stained clothes into hot water, which is a mistake because the hot water sets the grease into the fibers of the cloth. Cotton especially absorbs grease readily and will not easily release it. The best thing to do is to saturate the dry fibers with liquid detergent, which contains a grease-cutter. If the detergent is safe for your hands, it will not hurt your clothes.
When you have a grease-encrusted cooking pan, remember this: do not soak it in hot water! That just puts a layer of water between the grease and the detergent. Put the detergent directly onto the dry greasy crust, let it sit for a moment, then scrub with a nylon scrubber. You will be amazed how much harder the detergent works when you have not coated the stain in water.