Is it normal to feel that your career comes to a halt when you achieve tenure?
Tip 1: Sabbatical.

This is a great way to get up to date and to drive in your research again.

Tip 2: Leverage the abilities of your students/postdocs to help you stay up to date. If you find a paper that you would like to read, try to find a student for which the paper would also be useful, let them read it and explain to you. If you find a new book/topic you would like to know more about, try to give a lecture or a seminar about that topic. 

Tip 3: Say no to more administrative positions, if ever appropriate. You've got tenure and you do your service as you should do you may say no. Some people may not like it, but this is ok - they will get over it.

There was a recent question here that was similar with some great answers, but I can't find it right now…