If you have suggestions or ideas you can post it in the forum!
Kits are fun, and possibly even educational
Kits allow you to avoid manufacturing (expensive, difficult)
Kits can be repurposed
Kits let you get to market fast and test the waters: you can always 'productize' it later
If branding is important, try experimenting with different colors of mask and screen, but always start with green.
If you're just starting, you can try getting your PCBs panelized (tiled) and then de-panelizing them yourself.
A legend is key, if you have space and the design is finalized, consider putting values on as well.
FR4 1 oz copper is just fine. If you need 2 oz for some reason, don't forget to put in thermals for the ground plane to reduce the chances of cold solder joints
Make your vias very small and hide them, people may accidentally try to stick/solder component leads in there (I've seen it!)
Solder pads should be large as possible, your layout software may allow you to change this automatically. Otherwise, edit the packages.
Some kits don't come with PCBs, instead a solderless breadboard and wiring diagrams are used. This is a completely acceptable trade-off.
Shy away from SMT parts: many people don't have good enough irons or tools. Through hole parts are best. Sure its a little tougher to design 'these days' but think of it as a challenge!
If you go with SMT try to stick to PLCC (comes with DIP sockets), SOIC (not too hard to solder) and 1206s. Very few people out there can do QFP, 0603, uSOIC, TSSOP, etc. Even then, your support costs may start to get serious.
Powering your kit can be a tough design decision: consider USB power (5V 100-500mA), AA's or AAA's (be sure rechargeables are OK as well as alkalines), or 9V. DC jacks are also a good idea, use the mechanical switches built into most to break any internal battery connection. If you're using DC jacks try to stick to 7-12VDC center positive 2.1mm types, a full-wave or half-wave (a 1N4001) plus a large capacitor can save you from explosive tragedy.
A power on-off switch is a good idea, a 3mm 'power good' LED is even better.
Using parts from discount/surplus shops can cut costs, but always have compatible parts that you can buy from a big-name house like Mouser, Digikey, Farnell/Newark, Allied, etc. Use catalogs and findchips.com to get good prices.
Kits should have instructions available online so that people can see them before ordering.
If possible, include instructions with the kit too, or at least online
Schematics are a necessity with the kit, and should probably be available online but isn't as necessary. The chipset used, though, should be made public, along with any datasheet links.
PDFs are the preferred method of distributing documentation
You can use Instructables to host your documentation
Any microcontrollers should be programmed ahead of time! (The only exception is if its a development board and the microcontroller is a spare part) Imagine my surprise when after buying an AVR programmer kit I found that I needed to have an AVR programmer to make it work. Umm....
Batteries do not have to be included, unless the battery is rare, difficult to find, or very expensive to try to buy in one-offs. Examples of rare batteries include coin cells, lithium 3Vs, AAAA alkalines. (Read the advice on powering it for more hints)
Cables do not have to be included, unless the cable is strange or a one-off. For example, serial cables, USB cables, every day power cables are pretty common. 10-pin IDC cables? not so common. It may be reasonable to have them build the cable but mechanical/cable problems are notoriously difficult to debug so I don't suggest it.
Kits should include everything else necessary so that once it is built, it does -something- even if there's more possible functionality. (For example, you shouldn't have to go to radio shack to buy a 9V battery clip if thats a necessary part.) The only possible exception is wire, which is pretty common.
If you have SMT parts, especially resistors or caps, you can use a highlighter to color in the back of the tape to make distinguishing them easier for the user. Include extras, as they are easy to lose!
USPS priority mail is a 'sweet spot' for shipping in the US: 2-3 average service for $4-5. If your kits are light and nearly all orders weigh under 1lb, you can get stamps for priority mail postage and just stick them on. If you go online, USPS allows you to print labels with postage from a web-browser through the click-n-ship service and get free Delivery Confirmation (kind of like tracking). Another nice thing here is that you can get a whole bunch of free packing materials when you use this service. Yum!
International customers can be a significant source of your base, consider shipping to them! Air mail letter post is very inexpensive but can get 'trapped' in customs for a month or more. USPS Global Express mail is pricey but has a tracking number and is insured. I offer both, but each option is good. (Maybe try maybe shipping $50 or less packages Air letter?)
* If you are handy with simple layout software (like word, illustrator, etc), try making up 'template' documents for printing out labels, forms, etc. For example, I used Acrobat to create a customs form PDF that makes printing out international customs documents easy: just cut & paste the address and modify the total if necessary. The less handwriting you can get away with, the better.
Yahoo shops is an all-in-one solution that is commonly used. A nice intermediary.
If you have the mettle, try running your own webcart - I use ZenCart/OScommerce but whatever works for you is the best solution! Upsides are high integration and custom look. You can also get a merchant account. This is a good idea if you are good with maintaining web software and/or do thousands in business a month.
Forums seem to be the most popular method, they are easy to install and there are even free services.
Watch for trends: if 3 people suggest something or have the same problem, its something you'll need to fix
Half of your customer support emails will have to do with shipping, just the way it is!