Well, the title says it all. If you want your clan to succeed, to prosper, there a a few key points to helping you towads this goal....
NOTE: These following points are all just my views, of course there is many ways to run a clan, I just posted some of what
I think are necessary. And please, if you think I left anything out, please post and I will update :).
The Golden Rule:
MkWii was created for people to have fun. Clans tend to get too caught up in win/loss record which makes the game no longer fun. Winning isn't everything, and a great clan knows how to take wins as well as losses. Abide by this and you are well on your way to running a great clan!
Activeness:
The most important factor easily. The key to success for any clan is to stay
active. Without active members, it can be very hard to organize races etc, to contact anyone, even to get to know them. You should make sure when accepting applicants that they will be active in the future, and willing to devote some time to the clan. Also, it's good to know when your clan is 'full'. Very big clans can get very impersonal, so unless that is what youa re going for, I advise that you set a limit beforehand.
Thread:
It may not seem like much, but an attractive, appealing thread goes a long way to attracting prospective members. So take the time and set out your thread nice, get a banner in there if possible, and some colours wouldn't go amiss. Make you title catchy if possible, but try not to include any words like 'New' , for it is hard to get them removed when your clan is no longer 'new'

. be sure to mention all the good things you clan has to offer. First impressions are
everything, so make sure yours is good

Make sure you post everything you think is relevant to your clan e.g. Rules, How To Join etc.
Facilities:
It is a good idea to have a Clan Chatroom. These make it a lot easier to organize things, and help things run smoothly. It is also somewhere to talk to your members without the hassle of refreshing your page every ten seconds. I recommend getting a chatroom on
Chatango - Add Free Live Chat to any Webpage . They are free, and very easily accessible and easy to use.
Also, if possible, a website is a HUGE advantage to a clan. It offers and adds so much more to a clan, things like calendars, forums etc. It also makes your clan a lot more personal for the members. You can have clan games, music etc. *Also good for attracting members.
Participation:
It is good to get your clan participating in clan wars and tournaments as soon as possible. It's good fun, it's a proving ground, and it helps to get your clan known, again helping to attract more members. Try to organize a good few clan wars against other clans, I find that everyone improves a lot when racing vs other ones. Try not to make anything to serious though, after all, everyone got the game to have fun. Of course the basis to any clan is clan vs clan racing, so get up and go get 'em

. Try to get everyone to participate in races, no one wants to be left out, so let everyone in on the fun.
Some threads I recommend for good clan racing:
Grand Team Tournament - The GTT, a big inter clan racing tournament, every Wednesday and Saturday, unbelievable fun and great competition.
Clan Territory Wars - Good way to get some more excitement into your clan, with these intense clan races.
Tournament Schedule - Check out all the tournys available, the more clan racing, the more fun :)
Attitudes:
An important thing in Clan Leadership, is to know when you need to be serious, and when not to. Everyone wants to have a good laugh, but if that laugh comes at the expense of others, it's time to step in. If you find one of your members is quite offensive or rude, I suggest having a word with him/her to see if you can sort it out. In the end though, that bit's up to you.
Co-Leaders:
It is always good to have some people you can trust around you helping you make the important decisions. When you have to go away for a bit, or unavailable for a race, it's good to know the Clan is in safe hands

. Try designating responsibilities eg Finding Clan Wars, Setting up in-clan tournys. It is key here to know that you can trust your Co-leaders essentialy.
Decoration:
Now these are purely optional, but it always looks good to have a good clan Signature, and even a banner. Also, to race with a clan tag on your mii on WiFi is a good way to recognise other clan members and to advertise.