I find it's easy to upsell if you have something to sell. As Chestin pointed out, if you can give meaningful advice and insight while discussing it with them, they're basically sold. I recently did a quote off a referral where they had already hired a company, but were impressed not only by the fact that I was a student, but that I was so knowledgeable and would be doing the work myself. They really liked that I knew my stuff and that I could offer personal service, rather than sending some other guy to do the work (I was also $10 cheaper for the mowing even though I quoted above my usual price since they live a little far from home).
Before I started this job, I read an entire book called "Lawn Geek" by a guy named Trey Rogers. When people have specific questions, I often refer back to it. It's important to have the knowledge of what you do, since that's a great selling point and helps you get referrals through the higher quality of your work.