You can take any or all of the following approaches:
1) Get a Business Partner that is Established and Well-Respected in that Target Market
For example, let's focus on the service offerings for real estate.
If you have no clients, it's tough to get the first one.
But if you got a well-known broker using your software, and it's good software, others will take interest, and you have instant 'street cred' because a well-known and successful person is using your software offering already.
Further, if you have that broker helping you to develop the 'tools' that all brokers need, your software offerings will be that much better.
Finally, having the broker as a partner means that if more brokers use the software, that broker will make money as well - and they'll help promote the software.
Offering custom application development to the broker is a very nice 'carrot on a stick,' and you can approach such a person, if you have a personal connection, at first as a mentor (i.e. ask them to evaluate and help improve - they can use the software for free).
You have to be careful how you structure such an agreement - but that's beyond the scope of this question.
The key here is to break into a specific target market and gain credibility.
2) Basic Online Marketing - Web Site, Blog, Social Media and SEO
An SEO'd web site with a demo and good social media strategy is also a 'must.' This is basic and straight forward. A good web site can do wonders.
Make sure you do some keyword mining, SEO'd blogs about the subject, integrate the blog into your web site and social media - make sure you have clear path to 'attract, convert and attach revenue to those conversion.'
Of course, there are lot's more ways - but the above 2 can be pretty effective and are great starting points.
3) Get a Mentor and Ask for Help
Again, if you know someone in the target market, use that to your advantage. Take them out for a coffee, let them know you want their advice on how to market your product. Don't try to sell to them, get their advice on how to sell to their peers - this is a great way to build allies and support network.
Remember, that a lot of big companies started small, with just one client (i.e. 'Apple Computer') - everybody has to start somewhere, and people helped them along the way.
4) Put on Seminars
Put on seminars and provide some useful information to your target market. Look for associations, user groups, MeetUp groups etc.
Offer attendees a free consultation on how they can improve [insert relevant topic here], provide value in advance, build awareness and positive relationships.
There is a saying, "If business isn't walking through the door, then you have to go out and get it."