It goes without saying that you should be prepared, of course! These are just a few tips that I have found from my own experience working with retail shops and galleries.
DO your homework. Go on-line, visit the store is person, sort of do your detective work first to get a feel what the shop is like and if your work would be a good fit for the shop or gallery.
DO get a name. Of the shop owner, gallery curator or manager. Find out who you need to contact.
DON'T expect to just walk in without an appointment and get to be seen or to show your work. You won't get anywhere this way. Most galleries will want you to make an appointment to show your work, and that is, if they like what you make.
Which brings me to my next point; DO have a line sheet or catalog with you on your first visit. This way you can drop it off, and leave it for the shop owner or gallery director to view at his/her convenience.
DO follow up with a phone call or email. I found that most gallery curator's are very busy, and reaching them by email might be the best.
DO have your contact information readily available for the shop owner. If in person, while dropping off your line sheet, make sure you have links to your website(s), and full contact information. If email is your first contact with a shop, make sure to include photos of your work, your retail prices, your website and social network sites, and contact information. Make sure your photos look clean and professional.
DON'T just drop off your catalog and wait to be called. These galleries get many submissions every day. Make sure to call, email; just follow up.
DO learn the lingo. If you don't know what a line sheet is, or turn around time or net 30, net 60, I suggest taking a class for knowledge about the terms used in retail.
Creative Live has many classes to choose from on this topic.
DON'T put all your eggs in one basket. You may not get a yes at the first or second gallery you reach out to. Remember to try, try, and try again.
DO make a list of shops and galleries that you can contact. Let's face it, it's a number's game like a lot of things. If you want to get into retail shops or galleries, you must do a lot of footwork, and DON'T give up!
I must add this disclaimer: These are just basic steps that I have taken and have worked for me. My hope is that you will get something out of this post, but know that I am not a marketing specialist, and your results may be different.