Some tips:
Listen to top sales people on calls. They'll often have an ear for what people are most interested in and be able to handle objections in conversation. High-level sales orgs will record calls; take advantage of them.
Learn the product inside and out. If you know how it works, what's cool about it, and what sucks about it, your writing will be way more effective.
Be as clear as possible in your writing. Your customers are usually busy professionals who can detect bullshit--what works for selling diet pills to morons won't work in B2B.
Examine customer testimonials. If an enterprise client says "I love that this SaaS saves me time doing X" then you should have content that says "this product saves time doing X."
Study your customer. B2B SaaS orgs often have the advantage of knowing which roles in an org make buying decisions. Sometimes they know the exact person.
Check out SaaS sales funnels from other companies. Sign up for emails. Book yourself spots in seminars. Read white papers.
Check out the book <strong>They Ask, You Answer by Marcus Sheridan</strong>. It's not a copywriting book exactly, but it will really help orient you into what you're doing.