In addition to the out-of-sample testing, I would suggest some bias-correction measures, which are rooted in the information theory.
For example, you may consider something like this:
BCP = (P * SQRT(T)) / (D * D)
where
BCP is bias-corrected performance
P is the raw in-sample performance, such as Sharpe ratio
T is the number of trades
D is the dimension of the model (6 in your case)
So, in-sample, you want to find a set of parameters which maximize BCP.
For motivation behind this formula, take a look at the Akaike Information Criterion.