R help - May 2005 - lme How to validate a model with a validation set and a test set

Dear all,

I am still trying to fit a model for habituation with lme (Dogs are 
trotting on a treadmill once a week during 4 consecutive weeks).
I have maybe found a good one.
In order to validate it, I would like to split my data.
I have 28 dogs, I am thinking to build the model with 20 and use the 
remaining 8 to validate it.

Is it a good method to validate the model?
How can I do this?

I would like to use the data from the first 2 weeks as input and see how 
good the model is to predict the data from the last 2 weeks.
I tried to use predict but this function does not take into account the 
data from the first 2 weeks. I can only get the results for the population.

I would appreciate any suggestions for this

Thank for your help

Laurent