Check out the fares. Use the internet. Ask the ticket office (but remember they make mistakes).
Look for special offers - booking the previous day can be cheaper.
Check the price of season tickets.
If you travel first class check the price of upgrades - at weekend it's usually about 5 to upgrade from standard.
Consider taking a meal on the train - especially if you travel first class. you can usually use the restaurant car with a standard ticket.

If anything goes wrong with the journey fill in a complaint form. Try also to get a note from the conductor on the train.
You can often fill in the form on-line and it's an easy way of getting vouchers for money off the next trip.
More imoprtantly it also might make a difference to the attitude of the company if they get a lot of complaints. 

National train timetables