What do you think about these two things of a Domain name?

try using Domain Search - Domain Name Generator - Domain Name Search | Nameboy.com for some variations might help find a nice one

If "affiliatemarketing.com" is taken, i would use "affiliatemarketing101.com"

If my keyword is "how to train your dog not to dig holes" then i would never use it in my domain. I would still use "dogtraining.com" or "dogtraining101.com".

Depends also what you are doing with this domain. If you use it for a blog, then your domain don't need to be your keyword. Search engines also look for on-page SEO and not only domain name.

I've found that as long as the domain name "scans", it isn't a problem until you exceed 30 characters. That once was a limit on actually registering a domain name.
By "scan" I mean, can somone understand the words in your domain name? The search engines need to do so as well, but don't need hyphens (dashes) to separate words. One of the problems with long domain names is when you start using them in email, like your newsletter sequence, or with some online forums.

When you use a long domain name in an email or forum that doesn't allow hyperlinking, you want to make sure the name doesn't "wrap" to the next line, You see them all the time, somedomainname.com and then on the next line are more. This is called a broken link. If the person clicks on the highlighted text, nothing happens but a "not found" error page. Thus, if you are entering a long name, make sure the total URL will fit on a single line (say, 60 characters max.) and put it on its own line to make sure it doesn't get broken.

You want your domain name to be keyword rich. If you have a name like domainnamesforadollar, you should be able to get a page one ranking for the keyword phrase "domain names for a dollar" but you will probably be way down the list for someone searching for jsut "domain names" (particularly if they actually enter the quotes when they search). Enough off site SEO and you can probably rank well in the shorter version also, but it really depends on exactly which SEO methods you are using. Making sure that your pages have the proper tags goes a long way in helping your ranking for various keyword phrases.

The exact keyword in your domain is always preferable even if it is long. If you DO get a long domain name make sure it is a long tail keyword that is actually getting searched. In my experience you can still have some success ranking a site whose domain has the keyword plus a few words but its not nearly as effective as just having the keyword alone in the domain. Dot coms and Dot orgs have also been good to me so check for those first. If the domain you want is taken then I would search for another long term keyword using the original keyword as the starting point. With a little searching you can almost always find a good long tail keyword to use for a domain that no one is using yet.

For ranking purposes you can rank for anything you want, you don't have to have the keyword in your domain name.

Provided you have your keyword(s) in your title, and some good on-page optimisation then you'll rank okay for low competition stuff, once the competition get's higher then you'll need quality backlinks!

For really low competition stuff, having the keyword in the domain name can help you get an easy ranking however when the competition gets tough it doesn't really matter so much because it's all about off-page SEO.