It's my opinion that in order to make a good top, you need to be on the bottom first.

Fucking a guy should be as much about giving him pleasure, as getting it for yourself.

One thing that you can do to enhance your partner's pleasure is to use longer strokes. A lot of guys tend to slip their cock inside their partner, and just use quick short strokes. Speaking from personal experience - I really enjoy feeling a guy's shaft sliding in and out of me.
If you're a guy with a sizable endowment (congratulations), or a guy with a partner that doesn't get fucked too often and is really tight, there are a couple of things you can do to make things easier for him.

One is to first loosen him up. You can rim him or you can finger him. Select a lube that you prefer if you're going to finger him. Work a single finger up his ass and see how he reacts, then try two, if you're feeling adventurous and so is he, try three. Don't move the fingers in and out too much, let your partner get used to idea of having something inside of him.

Once he feels comfortable, try slipping just your cock head inside of him. As you feel it slip in then stop and hold it there for a while. As your partner adjusts push in a little more. If you see your partner wince then stop and hold it there, pull out just a little if he needs you to.

This way you should eventually be able to get most if not all of your cock inside of him. Once you do, use long strokes for the maximum amount of pleasure. Try not to pull completely out of him while fucking. That can cause air to get packed inside your partner's ass, and can lead to some rather embarrassing results. Though slipping the cock head in and out of your partner's ass can be a wonderful way to tease him.

Don't forget about your partner's cock while fucking him. If his cock is accessible play with it - unless he's like me, and a couple of quick strokes while getting fucked will finish him off. It can be fun to slap his ass playfully while fucking him.

Fucking can get pretty routine. If you're used to one or two positions then I urge you to try new ones. I won't suggest any here, because there are just so many that you can get into. I will say, though, that I love nothing more than throwing my partner from position to position while fucking him, or getting fucked. Encourage your partner to try new ways. Also try different locations, other than your bedroom or bed. Have you ever been on a secluded country road and bent your partner over the hood of a car while fucking him? On the balcony of your house/apartment? In a sex club or in a hot tub? Have you ever done it on a hillside while overlooking a beautiful view, or on a beach on a starry night? You can try different places around your house too. As popular a stereotype as it may be - I don't recommend airplane lavatories, they are usually way too cramped, and if you think the flight attendants won't notice - well don't be surprised if they offer you a cigarette after you come out.

When it comes to lubes there is a wide variety of choices that are available to you. Everything from spit to tons of commercial brands. Personally I'm not too fond of commercial sex lubricants. They tend to get too sticky, or they dry up, or they can be way too slick, or they are a big pain to clean up. One lube that gets overlooked quite often is lotion - Keri Silky Smooth Formula is the best I've ever tried. There are a lot of water based hand and body lotions out there.

Personally I prefer lotion because there is a lot less stuff to clean up, they tend to absorb somewhat (where sex lubes do not), and they provide just enough lubrication to make things comfortable, and not so much that you feel like you're fucking a moist rag.

There isn't much more I can say on this subject, except that creativity is the most important thing here.