Well to be honest you are going to need some kind of marketing budget even if its a small budget. How many leads do you need before you gain a sale. For instance, do you need to generate 100 leads before you ultimately produce one sale. Knowing this will help you understand if your marketing tactics are working. How will you capture leads?
I would begin with a website, doesn't have to cost a lot of money to set up and maintain. You can use the website to produce information for prospects and capture leads. What are the problems that your customers typically face? Create content that answers these questions. Join the groups that your prospects join and participate. Write articles for the trade magazines and association they are involved. Create a newsletter that will help you keep in touch with prospects and keep them engaged with your company. Inbound marketing requires that you create content for your prospects and put it out on the web for them to find. You can create videos demonstrating your software. Create slideshows with slideshare demonstrating your software, webinars, podcasts and a variety of other content pertaining to your software, including white papers, special reports, ebooks and much more.
This strategy take time. The first step is to create a marketing plan for your business. I would create an one year marketing plan. Then research how your prospects go about obtaining information when then begin looking for software to purchase, what are their main concerns and address those concerns with your marketing material. Use linkedin or other online networking groups and your current customers to ask questions and find out what their needs are and how they go about researching solutions to their needs. Then find ways to go about addressing those needs. If you are not a writer you can hire a writer for a reasonable cost on a project by project basis. I would be aware that the cheapest writer is not always the best writer. Find a writer who understand your industry and can convey the information in a way that is in line with your how your company presents itself.
Begin by creating your marketing plan so that you can plan these activities over a period of time and spread the cost. Don't forget to create a way to measure your marketing activities.