Great thread!
When applying for any job, always tailor the resume and cover letter to that specific job. Use keywords from the job posting to get through the automated program and HR. If the application allows for one, ALWAYS make a cover letter! After an interview, it is wise to send a thank you email to the interview panel or at least the main hiring manager. Always be courteous and think before opening your mouth!
How you interview is the most important. If you are nervous, don't worry. It's more about how you talk about your knowledge and demonstrating you know these things and aren't talking out of your you-know-what. Also if English isn't your native language, try to speak clearly because a few people I have interviewed I couldn't say yes to because I understood maybe 2 words the whole 30 minute interview. I had no idea if they actually understood us or if they answered any questions appropriately Most of the time GIS jobs require being a liaison or coordinating and interfacing with a lot of people so you need to demonstrate you can communicate clearly.
I always start with "describe your experience with GIS..." and talk about the jobs and education relevant. Don't just read off your resume, I have it in front of me! For every job experience that you have, reference a project or map or something you did there that was significant or also describing your main job duties but going into more depth than the one liner in the resume. It's a huge deal to go into details to show them that you actually know what you are talking about.
A portfolio is HUGE! If you can show maps and reports and other work that you did, bring it! Even in phone interviews when you can't show them anything, talk at length specifically about different projects and maps. You want to prepare ahead of time which projects or things you did that are relevant to this particular job. Have something for every job just in case they ask specifically about one.
I don't like putting candidates through tests because it makes everyone nervous and the nervous really nervous! I do, however, ask a few technical questions that are oddball but easy for the experienced GIS professional to answer to see if they actually know their stuff. Sometimes the oddball is defining what a layer file is or something about projections, stuff that would catch a smooth talker off guard or someone that is lying (had several of both kinds). It usually depends on what I see in the pile of resumes and the level we are hiring at. I pick a few random things out of the resumes whether it's something I saw that was questionable in one resume or something common in all. If you don't know the answer off the top of your head, the best answer is that you don't know and to expand on what you think it might be and maybe reference something similar that you do know or at least how you would find out the answer. The best guy I ever hired couldn't answer some of them but the way he talked about his knowledge and the way he approached these questions made me confirm that he was intelligent, he was fresh out of college so I expected some "I don't know" answers. I just want someone that can own up to their lack of knowledge and be willing to research and learn, this goes for all skill levels.
For education, I like to see a BS in Geography and if they had taken GIS courses, whether from ESRI or college, because it's really good for getting into the spatial way of thinking and gives them a good base of knowledge. Masters in GIS is a plus too. Education definitely is a plus overall but not a deal breaker. The job experiences and how they interview are the most important. I want to see a real world application of GIS and for fresh out of college students I want to see enough base GIS and Geography knowledge to jump start your career.
Certifications like the GISP don't matter to me all that much. It just means you jumped through hoops and paid money to prove you know the basics. Not everyone is a good test taker and not everyone is willing to pay for it, so it's nice but that doesn't prove much to me.
Also dress appropriately. I like to see suits and combed hair! And don't go overboard with perfume and cologne, it impresses no one, especially those allergic to it.