When people want to guest blog and post on article sites, they usually want to talk about themselves or give themselves the best possible backlinks from the most ideal positions. The problem is that pitching a product or service probably isn't interesting to an audience. You need to be creative. Blog readers and community leaders who approve these articles want something that ads value to their readers so keep that in mind.
What you need to do is take the standpoint of a blog reader of the site or community you are writing for. When you create the article, it may not have anything to do with the topic of your site, but you will be able to include the keyword rich backlink that you are looking for. Here are some examples of creative ways to tie in a keyword rich backlink to an article that has nothing to do with your site or products. It does however give you the link you want to your site and gives the audience an article idea that may be interesting to them.
I'll use two general products that tend to be the same as every other product or service to consumers, Diet Programs and Insurance.
Diet Programs and How to Adopt a Cat from the Animal Rescue League
Adopting a cat probably doesn't have a lot to do with a diet program or pill. You can however talk about questions to ask the people adopting the cats including things about overweight cats who may need a diet program. Sure it isn't about a human diet but you get the link off the keyword phrase diet program that you were looking for.
Diet Programs and Finding a Job
Some people may say that companies tend to hire better looking people, I have no data to back this up, so you could always write an article about how to improve your ability when looking to find a job. One of the things you could talk about may be starting a new diet program. Other things you could mention could include practicing interviewing, fixing your resume, learning new skills, searching all of the job sites, etc...
Diet Programs and Learning Spanish
If you are writing an article about learning Spanish, you could always talk about using common words and some odd phrases like Diet Programs, Fast Food, Where is the bathroom and how does my hair look? Then you can link off of the keyword or put the keyword and phrase in the description and link to your site from there.
Insurance and a Kids Tea Party
You might not think that you would need insurance at a kids tea party, so why not give the kids each a role to play. One can be a princess, one can be a business owner, one can be a Real Estate or Insurance Salesperson, etc... Another option is to create tea party manners and give recommended compliments like this tea is so gorgeous that I hope you have insurance on it. You can then put in a joke about annoying brothers always breaking things.
Insurance and Teenage Dating, a Father's Perspective
This is always a fun one. You could do anything from the Father saying I hope you have a good insurance policy and reference bringing his daughter home to late. Another option are things to ask your daughter's date like do you have car insurance in order to make sure he is responsible, etc... By adding the questions in to other questions you can make them flow and look like they belong. You could even make it about joke threats from father's to suitors for his daughter and tie it in letting him know his health insurance better be up to date if his daughter is late.
Insurance and the different types of pizza cheeses
If you are writing for a cooking or food blog, why not make a joke at the end by saying something about "with this much cheese your stomach will need its own health insurance". You could talk about using saffron or other expensive ingredients and make a joke about the dish being so rich it comes with its own insurance policy, etc...
The thing with guest posting is that even though you love your products or company, audiences probably don't care. What you need to do is write an article that their audiences would like and keeps within the theme of the site. There are always ways to tie in the phrase you are looking for and get the backlinks you want. The most important thing to remember is to give the readers something that they can take with them.