Rich, these are good guidelines you list in terms of evaluating nutritional companies. However, the original poster asked specifically about sampling. So I will attempt to address that aspect.
In order for any type of sampling program to have long term sustainability and hence success for the people involved, here are the characteristics that should be in place. First of all, the product must provide a fairly immediate impact, or else the person receiving the sample is probably not going to order it. There aren't many supplements that work fast; typically one must use them for at least 30 to 60 days to notice any type of health benefit. Secondly, the COMPANY should send the sample out to your prospect. This is implemented in such a way that you receive credit should the 'samplee' order anytime in the future. (example prospect watches a video on your replicated site and enters their information to receive the sample); they are placed in a holding tank until they order, at which time they become part of your group. If the product is dynamic enough, the prospect ideally would just order an entire unit, again through your site, without any intervention on your part. If they don't, then here is what I think is the key, in terms of follow up: the company provides a call center fully staffed to answer questions about product, compensation, SAMPLING, etc. The call center then enrolls the prospect on YOUR behalf! And finally, the samples should be fully commissionable. In other words, an existing affiliate purchases a certain number of samples each month, which the company holds until notified of someone wanting a free trial (totally free to the prospect, shipping is built in to the sample cost). It's a win-win for the affiliate and the prospect in that the affiliate does not have to inventory samples (although that option would be available if the affiliate wanted to personally hand out samples on a local basis), AND the prospect gets to try something totally free with zero shipping costs and no credit card required.
This type of sampling program is a more professional way to do it in my opinion. Most sampling programs I've seen in the past are quite haphazard. This type of business model is more duplicatible, hence more apt to create long term residual income. There is only one company that I know of which incorporates all these features in their sampling program...
(btw, I do not actively promote the company, but have tested the sampling program, and it does work)