Rewarding your players directly can be a dicey proposition. Funding significant rewards, for one thing, is difficult unless you and the officers already have boatloads of gold in the game (or you are independently wealthy and have plenty of cash to spend on items from the Blizzard store). I'm assuming neither of these is the case, so where is the real or virtual cash going to come from? That's an issue you need to solve up front.
Fortunately, once you hit level 5, your guild will have Cash Flow. After that, you'll always have a moderate source of capital to invest in rewards.
Drama and incentive
The other issues are more drama-related. Who gets rewarded for what? If someone kills 50,000 critters for the guild achievement, how do they prove it? Blizzard's UI doesn't track contributions to guild achievements. In many circumstances, you'd just have to take someone's word for it. That can lead to deception, tattling, jealousy, and all sorts of nonsense.
For guild leveling, it is possible to see who contributed the most to guild XP per week. Just look at "Guild Activity" in the roster panel. It is tracked on a weekly basis, so by checking it late Monday night you will see who earned the most XP that week.
However, the person who "wins" is inevitably going to be the person who has the most time to spend online. That person is likely to have a large contribution to XP regardless of rewards. Offering them further incentives will have a small impact on the overall XP earned. Also, when the same person or persons wins week after week, it reduces the incentive for other players.
However, if you reward the entire guild rather than individuals, it spreads the incentive and encourages even the players who don't have much free time to do their part. I recommend this strategy for both leveling and achievements.
This reward method encourages cooperation rather than competition. Since Blizzard chose to make most of guild leveling a soloable process, it falls to officers to make it as collaborative as we can.
Here are some ideas for rewards that are both fun and reasonably inexpensive.
1. Post a silly pic of yourself on the web site. This one is simple and memorable. Tell everyone in the guild about a silly or embarrassing picture of yourself. This could be a real photo or one you stage. Say you will post it if they can reach a specific level or earn an achievement in a certain amount of time. Few things motivate people more than an opportunity to make fun of the guy in charge.
Alternative ideas could be promising to sing a certain cheesy song over Vent, or dressing your character in a ridiculous outfit and dancing on mailboxes for tips.
Best of all, this type of reward is free. The only cost is your own mental and emotional well-being!
2. Hold a raffle. You're not directly rewarding everyone, but a chance at something cool is often better in most people's minds than a guarantee at something unexciting. Even if people don't win, they're happy that they had the opportunity, and they're pleased with you as the one offering it. They know you don't have to do anything of the sort.
For leveling, you can buy one item to raffle off per level. Start off with simpler things and work your way up to bigger and better loot. For achievements, you can target specific ones and offer themed raffle rewards for each.
Execute the raffles after guild activities you want to promote in order to boost attendance. (Just don't exclude those who weren't able to attend from being eligible unless you want a lot of complaints.)
3. Start a "make-a-wish" campaign. Let members make suggestions for things they'd like the guild to do. Leave it completely open-ended. It could be attacking a certain enemy town, going after a specific (player) achievement, running an old raid for transmog gear, or even something like holding a real-world party at an officer's house. Create a poll and let everyone vote on the one they want. Start with the one that gets the least amount of votes (above zero) and work your way up the list as the guild levels.
Trying to inject more fun into leveling or achievements is commendable. However, don't lose sight of your main priority in the early stages of the guild, which is making sure everyone has plenty of organized activities to keep them busy. If you make sure everyone is having fun with PvP and raiding, then you will retain members. Retention adds up to achievements and levels over time.
If anyone else has a good idea for a reward system, tell us about it below!
Officers' Quarters keeps your guild leadership on track to cope with sticky situations such as members turned poachers or the return of an ex-guild leader and looking forward to what guilds need in Mists of Pandaria. Send your own guild-related questions and suggestions to firstname.lastname@example.org.