This is the most important for Ranking a new domain. If you fix Bikes. You want Fixbikes.com of course. If you only work your local area FixBikesFlorida.com FixBikesJacksonvilleFlorida.com is a big jump start into getting calls online.
Fix-Bikes.com will work but to many Hyphens can be considered spammy. I don't think people type in Hyphen domains honestly. One that same way of thinking though I believe people just use their browser search engine to fins the site they wont. So knowing a domain is not important to most anymore. You can import your bookmarks to any computer. It is like phone numbers no can remember phone numbers any more because phones have been doing it for the past few years now.
3. Domain Name Memorability
As I stated above I don't really see this as something to worry about. Unless you are wanting long term marketing. In that case have 2 Domains like Foursquare.com has 4sq.com one easy and one SEO ready.
4. Only Buy if you can get a top Level Domain
You only need your STRONG .COM
Depending on how well you know SEO already or how good you are at ranking you already know that some of these Do's and Do not's can be over come with tons of hard work but we can all agree that Buying the right .COM in the beginning saves tons of Money and Labor on SEO later. That being said You only need the .COM the rest you can buy later if you care that much. No one accidentally types .NET when trying to go to your .COM so don't worry. owning them won't bring any legit traffic to your site.
All that being said as we all know getting the right .COM is not so easy nowadays. So the next step would be .NET for me. Stay away from .INFO .WS .BIZ
5. Sub Folder or Sub Domain?
If you already have your domain and you want to add to it or you have a blog which is better blog.example.com or example.com/blog ? Sub domains are like starting with a whole new identity and Google don't trust it. If you apply everything listed here into your sub folder think about it like this you are adding more keywords. Like Fixbikes.com you can then add FixBikes.com/ Brakes or whatever you are wanting to promote or start focusing on.
6. Buying Domains with backlinks already and linking them to you.
Besides the fact that the links are more than likely completely unrelated and poor in quality. Alexa and Google usually devalue the links so bad it is not worth the trouble. You should stay focused on what you are trying to sell don't buy up anyone's failures.
7. Buying Your Domain Registrations into the FUTURE!!!
It can't hurt. I buy my domains at least Ten years into the future. I like to think that it helps but based on what the people at Google say it does nothing. The overall age of your Domain matters but buying into the future is not aging your Domain.