Give her a big hug and kiss everytime you leave as if it will be the last time you see her - I promise you that you won't regret doing so. And if you want to hug her, hold her hand etc during your visits, do it, don't just wait until it is time to leave, it will show her how loved she is.
Talk to her about anything an everything, ask her questions about her life you may not have known, simple things that you won't get to ask once she is gone.
If you can, take a photo of you & her now, it might be a painful reminder to see her looking ill once she is gone but it might not be and you won't know unless you have it. You can put it away if it does upset you.
Take her the paper, read it to her. Take her all sorts of things she likes, take your children to see her.
If you have an ipad or something she can use to read news and watch films on to pass the time when you are not there then take it. Will also keep you entertained as she gets closer to death and more of your visits are spent with her sleeping.
Visit as often an for as long as you can. You can catch up on sleep, work, social life etc once she has passed away. You won't regret spending so much time there but you might regret not spending enough time there.
Thats just from my experience of having lost two immediate family members to cancer recently... The first I didn't do as much of the above and felt sadder for so much longer, the second I did all I could and dealt with my grief better as a result and more peaceful.
I'm sorry your mother is ill x