I deffinitely feel my stomach drop as soon as I look at a bottle of Tussin, and I've been doing it for several years now...
The best thing that I can say to help cut down on nausea is (if you can handle the taste) to take it very gradually. This WILL mean that you will be at least a little high for 20-30 hours, depending on just how gradually you take it. I always do this when I'm taking huge doses. Example :
I wake up and drink 6 oz. (Tussin Max Cough, approx. 90 mg/oz.) to start off with a buzz that will be maintained for the next two days. 2 hours later I feel it and drink 2 oz...2 hours later, 2 more oz...2 hours, 2 oz., 2 hours, 2 oz., and so on. I'll do that until I can instinctively feel like my buzz will not just be prolonged if I take more, but rather that I am deffinitely getting much more high by the minute. Usually at that point I try to drink as much Tussin as my stomach can hold (usually 4 or 5 oz., never more than 8) *. Within 45-60 min. you should be so high that you don't know what nauseous is. Take this time to find a chair that you don't mind sitting in for the next 4 hours, because you will not be leaving it once you sit down. Smoke some pot. Have your wife (or whatever...) sit next to you to try to ask you questions about what's going on (write that shit down!)
I've managed to get down 31 ounces this way without ever getting sick, whereas I usually feel ill and maybe 10% of the time puke if I drink 8 oz. by itself.
Hope this helps.
* When I first posted this the "eight close parentheses" showed up as rolling eyes. I meant to say "...never more than 8 close parentheses"