A community is only as strong as the engagement it has with its users.  Here are 25 tips to help you build that engagement.

1. User-specific offers. Keep it personal.  Try to target individuals rather than segments.
2. Group buying / offers. If buying something is considered to be the pinnacle of engagement then encouraging group buying among friends (for cumulative discounts, etc) might be a good idea. Groupon et al have shown how popular this approach is with users.
3. Free upgrades. It's much better to make existing customers happy than spending money trying to acquire new ones.   Why not give shoppers the chance to upgrade products and services for free, either at the point of purchase or out of the blue?
4. Extended features. Think about some of the games you loved playing as a kid.  You do well in the game and you unlock special features, which encourages you to play more as the faster you perform, the faster you get the new features. Think about that… maybe you can create features that the highly engaged can open up?
5. Mentions. We all like to receive a pat on the back. Recognition can make people feel special.
6. Exclusive / special events. People love the VIP treatment.  If you can make your best users feel extra special then it will improve engagement no end.
7. Better products, services, content. Social media allows you to listen to what your customers want like never before.  You'll build engagement if you actually act on what they're telling you.  Arguably the best way to reward your caring customers is to finesse your products, services, content and websites to improve the overall customer experience. Tell customers that changes are based on their feedback and they’ll love you that little bit more.
8. Product development. There's no need to stop there though.  Rather than waiting for customers to complain about what already exists, why not invite your best customers in to co-create your next generation of products?
9. Priority shipping. How about offering same day or next delivery for the same price as standard shipping (or free)? Delight customers with superfast delivery and perhaps they’ll shout about it, and purchase from you more often.
10. Power ups. Many forums allow you to see who the influential users are.  They might not post the most, but when they do contribute, people listen.  Give these people extra goodies to thank them for their contributions.
11. Prestige. Status. Badges. On every community I've worked on giving people awards and badges based upon their activity proved a great motivation.  You may think a title or graphic on a website is not a big deal, but they really do motivate users.
12. A courtesy call. Social media sites are notoriously difficult to actually engage with person-to-person, so picking up the phone to talk to your best members in person is so unique as to be incredibly effective. A call out of the blue (or at an opportune moment) can be well received.
13. Print. I still like the feel of a book in my hands and have thus far resisted the urge to buy a Kindle.  A print newsletter to your best members is a nice way to stand out from the crowd who only offer email communications.
14. Remove ads. Few of us like adverts so if you could offer people a way to use your site without having ads then that'd be a great incentive for them.  There could be an incentive to remove the ads from your content in exchange for some form of engagement, which could be as simple as a new registered user account. Data may be more worth more to you than low-rent CPM ads, and it provides a strong incentive to sign up.
15. Private shopping. If you're a retailer this is a nice form of special treatment.  Open your store just for your best community members and pamper them.  If you're not a retailer think of some other exclusive treat you can offer.
16. Preferential terms. If you offer financing to your customers, maybe you can offer your best community members better tems.
17. Add ons. Likewise, most products now come with add-ons, be they functional add-ons or something like insurance.  Offer these free to your best members.
18. Gold card. Send them some kind of discount / loyalty card. A gold card!
19. Birthdays. You need to be a wee bit careful with this one so as not to come across as a bit creepy, but an individual greeting with some thought behind it can work wonders.
20. Vouchers and coupons. Another form of discount or special offer here, but if done in a nice and stylish way it can make the person feel special.
21. Exclusives. What do you do if you are a consumer electronics retailers and have a limited supply of the new iPad? I’d suggest that VIP customers skip the queue and are offered first dibs. This could extend to priority discounts, if you were feeling mighty generous (though you’d be mad to discount an iPad). It pays to play. The most engaged should be placed at the front of the queue, when you launch a sale. These people are the most likely to shout about your awesomeness.
22. Follow them. Tune into the most influential via Twitter etc.
23. Ad campaigns. Word of mouth advertising is incredibly effective, whilst crowdsourcing has revolutionised many industries.  Could your community members create an advert for you?
24. Product packaging.  Likewise, design is now an area that is utilising crowdsourcing very effectively.  Could your community design your packaging for you?  Think of the engagement if your customers have been actively involved in the production of a product.
25. Hire them. Some customers / users are so active, influential and into your brand that it might make sense to actually hire them

A few ideas there to improve community engagement.  Do you have any others?