1 - read several past issues of the publication in question. Editors do not like pitches that make it clear you have barely browsed through.
2 - most publications display their editorial calendar on their site so that advertisers can choose what issues they want to be in. Typically you can find this by doing a search on their site or asking for a rates package.
3 - Remember that publications typically put the issue together at least two months before publication. So if you want to make sure you have a shot at the July issue, get your submission to them (or letter of interest) by March.
4 - If you are emailing numerous editors, do NOT "CC" them. Editors do not like to see 'spray and pray.' Email each one individually, and treat each email like it's the only person you get to speak with.
5 - Do not bug the editor with follow up questions on the status of your pitch or submission. They know what they are doing. They will typically tell you how long to wait before following up, or when to assume they will not be using your submission so you can take it elsewhere.
6 - read the submission guidelines and make sure you are okay with following them. Some want first rights to your content, some want exclusive rights to your content. Make sure you understand what they will do with your submission besides just publishing it.
7 - like Steve says, do not take rejection personally. If they want you to make changes, and they don't literally harm you to do so, then make the changes. Likewise, be prepared to have your work edited without your permission and possibly without your signoff before publication.
8 - remember the these editors get hundreds of queries a day, and are constantly putting together, and tearing apart, editorial calendars and copy layouts to make room for this or that. They don't have time to tend to hurt feelings; they have a job to do, and that job is to please their boss, not you. Don't take anything personally, and don't treat them like they are sitting around waiting for something to do.
9 - never turn in a pitch with typos or poor grammar. That's just an indication of more to come. Hire a pro to create your pitches if you have to. Their title is Editor, but they will not correct your mistakes. Never make them work to publish your information.
10 - choose what publications you'd most like to appear in within your industry. The reason is that oftentimes, publications will not always accept your submissions if they see you writing everywhere else. You'd think they'd want a piece of the action, but it can be a turnoff to some publications.
The better your relationships with editors, the more you tend to get published, the more you tend to get to review the edits, and the more they tend to come to you with article requests. Networking with these folks correctly from the get-go is key to forming a relationship that gains traction.
Michelle Gower provides kicka** results for kicka** people at Gower Power Consulting, a WordPress training, mentoring, and assistance authority, in Raleigh, N.C. for the solopreneur, WordPress provider, and small business owner. She can be reached at email@example.com or at 919-228-8450, and of course, her wit can be enjoyed in full glory at http://www.gowerpower.com. Register for October's How to Build a WordPress Website course at Wake Tech Community College with Gower and Martin Brossman.