"Thank You" are two simple words that are quite important to use at any time, but especially in business. There is no excuse for abandoning this MUST DO step in any deserving interaction. If and when you skip this step, you align yourself with the risk of negative connotations. In today's fast paced and impolite society, using these words will help you stand out from the brusque and busy crowd.
Don't assume your gratitude is implied, or that you took care of it in an earlier email or phone conversation. Always follow-up AFTER another person has made an effort on your behalf.
You must say "Thank you" whether it took someone 2 minutes or 2 hours to: offer you their time; answer a question(s); connect you with another person; help you save or make money; give you an opportunity; send you a gift; share their expertise; or anything else that benefits you. Use the format that you find easiest to get the job done... It can be a handwritten note, a typed letter, a simple email, or a quick phone call; and in some cases, even a text message will suffice. Try to follow-up immediately, but if time passes, remember that it's never too late to express yourself.
No matter the form, it is the substance of a "thank you" and its intent that carries your message loud and clear.