When there is a technically challenging product and complex solution, how I keep the channel interested to provide the first level support?
The answer is quite simple - seek out resellers who add value, or in industry terms VARs - Value Added resellers. This type of reseller, and sometimes distributor (known as VAD, and common in Europe), love to sell technical products, as they make their money from services - design, installation, and support. They often the product revenue sale as "gravy" on top of the deal, and sometimes even a necessary evil.
VARs want to charge to provide first level of support to the customer, and deliver important leverage to a vendor's support organization. To ensure that the reseller delivers good support, you should give them access to training, knowledge, and tools. And measure their customer's satisfaction from time to time to ensure that your product is being represented well.
Believe it or not, many of these resellers will actively ask a vendor NOT to make the product easier to use, as they feel a complex product maximises their ability to sell services. I don't subscribe to this point of view, as the easier you make the product to evaluate, the easier for the reseller to sell. So you train your channel to move their services up the value chain and sell more 1st line support, design/architecture and consulting services.
Where this can come unravelled is when an organization gets greedy, and tries to cut out their reseller channel and go direct. Always remember that these guys are your partners, and they give your organization needed leverage. Can you really afford to lose that?