Hard to be specific without knowing exactly what you can specifically offer. However here are a few thoughts.
Make a list of all the benefits you can offer your customers and memorize all of them. Think of all the questions a potential customer might ask and have the answers ready in your mind before you call on them
Make sure you are speaking to the decision maker before you invest too much time selling your services.
People are busy so respect that, after your greeting ask permission to spend a little of their time explaining what you have to offer. If they say they are to busy ask to set up a later appointment at their convenience.
Always empathise with the customer. You should both understand and be able to convey to them how your services can benefit them. (If all you are concerned about is selling them something without caring or thinking about how it benefits them you won't be as successful.)
When giving your pitch try and lead the conversation by asking questions. If you find that they are the one asking all the questions and you are just answering you are not going to be as successful. However you want to involve them enough so that they do ask questions of the things you have lead them to think about. You want to help them arrive to where they are ready to make a buying decision. If they are not involved enough to have any questions or concerns (objections) they are probably just being polite and gathering information but not actually at the level of interest where they are considering making a buying decision.
When you are getting close to the point of asking for the order try asking questions that will get them to respond positively. Such as " Mr. Jones, can you agree that these promotional shirts could help you attract more customers to your store?"; " Mr. Jones, you would agree that these shirts would be an inexpensive and yet very effective form of advertising for your business, wouldn't you"
If they ask a question that could be answered by asking them a closing question in return try and do so. For example if they say can I get this in red? Don't say yes, but rather say, "would you like them in red? If they ask about a discount, don't just say yes, instead say, if I could give you a discount are you prepared to place an order?
Ask questions that either answer helps close the sale. For example instead of just asking, would you like to order some shirts, ask would you prefer to have your shirts delivered or would you prefer to pick them up? If they were considering a few different colors or styles, ask would you prefer the red or the blue? Questions like that are very effective, but relate it to what they have show interest in. If they are asking about quantity and price breaks you could ask, would your prefer the 50 option or the 100 you were considering? If they had asked about fabrics, you could ask would you prefer the 50/50 blend or the 100% cotton you asked about.
The most important thing is to always be honest and try and build a relationship rather than just making a sale. Care about your customer and their needs. If your number one concern is helping them, your sales will come.
And always remember it is always best to under promise and over deliver, rather than to over promise and under deliver.
One more thing, never interfere with them conducting their business. If that means you wait or stop to tell them to handle something that occurs while you are there, you do that.