Well, apart from the obvious, don't look at phones, tap your foot, maintain eye contact and so on, there is one very concrete and direct way of ensuring that you look engaged in a conversation.
I have found that people generally will participate in active conversation when you reciprocate in some way.
Generally asking questions directly related to the subject matter they are discussing and commenting in ways that are more than "yeah", "I agree", "Wow, thats cool". Inject parallels to your own life, give examples from what you've seen or read. These things will generally make you seem engaged even if there may be a lack of eye contact, or if you are looking at your phone. In a conversation don't be afraid to interrupt to ask a question, even if someone is telling a story. Oftentimes, it will not seem rude and in fact may indicate that you attribute importance to certain things someone says, which will definitely boost how engaged you seem.
In the end, it seems that the best way to seem interested is to actually be interested in what others have to say.