1. Get to know your users properly
Even if you don't have any users yet, you are building a product on UX by top UX design services, so you will need to do this anyway either this or that(how else will you know your market). Get specific as very few products are for "everybody" and not for all. You can always recognize different types of users that have a variety of demographics, needs etc. Also find out what makes each type tick (preferably by talking to them) and write this down (You can take this one step further to the positivity and construct persona if you like).
2. Analyze your Product
Understand the tasks, goals, aims, steps etc. that users will face such in your product. Map out the flow of tasks and to think how that translates to controls, screens, dialogues etc. in your system. Identify the typical scenarios (based on what you have learned from users!) and make sure that the flow makes sense for those. Think about what is going to happen in exceptions.
3. Get your Feedback early
You don't have any compulsion to have a finished product to get feedback. You can get feedback on lots of things, from stories to crappy drawings and designs on the back of a napkin or a paper. Again, lot of this stuff you will have in any of the way. You probably sketched some screens of your interface before you begin your coding HTML – show those to some people and make sure that you ask the right things to the right people so ideally you get feedback from real potential users, but if you can't get your hands on those and the things doesn’t work, you can also ask friends or even colleagues or any unknown expertise of the same field(those are obviously biased, but can still get you feedback on whether screens are confusing, buttons are not labeled clearly and many other things, etc.).
4. Repitition
Keep asking and trying for feedback as you go along. Refine the flow by rearranging screens, etc. where it's needed.
5. Use Social Platforms
If you are a web startup, you can get a lot of feedback from the web ie internet, some of it for free. Websites such as ‘five second test’ which are great, because you can post a screenshot and get feedback from a total stranger for free and also an unbiased one. You can also look into things like Amazon Mechanical Turk and ask unknown people for feedback on Twitter.
6. Use enumerated remote user testing
For less than 200 dollars/pounds/euros whatever you can get from a few complete strangers to use your website, performing tasks you give to them, while recording a video of their actions, mind and thoughts. This has lots of limitations (they are not real users and may be too experienced, not to interact with them, etc.), but the information you get from it has very good amount for money.
7. Get in touch with your users
Make it easy for them to get in touch with you, but also try to reach out to them when they talk about you on Twitter or any other social media site or something. You'd be surprised how willing to talk with some people are and how much valuable information you can get from them. If your site is live than you can even recruit people from among users of your site using popup windows (but target them properly so that you don't annoy and scare away users/customers. Services like ethnio could help). Offering a reward (a gift card, or a present for example) will make sure you don't just get only people who really love or really hate your product, but also people who just want a gift card or a free vouchers and are otherwise more "average" and representative.
8. Measurements
Define what indicates distinct levels of "success" for your product. Measure things like registrations, logins, active users, purchases, sales everything. You can only detect problems if you know what the problem looks like otherwise it's confusing.
9. Use A/B Testing
Once you launch, this can help you test the impact of changes and modifications you make. Be careful that you're actually interpreting the results right because it's easy to draw the wrong conclusions rather than the right ones. To your homework and define the correct performance metrics and set up the test in a solid way.
10. Get the expert's opinion
If you can afford it, it may not be unworthy to pay someone to give their expert opinion. I think this is most valuable for solving problems, rather than finding them and rectifying them. If you have identified and analysed a problem from user feedback or usability testing, an experienced designer might be able to help you understand why the problem existed and also would propose the solution.
Conclusion
A lot of this may seem like blossomy for a UX professional with a budget, but doing anything is better than doing nothing at all. The most important thing is commitment and if you can take some time to talk to users and think about how you are supporting them, that would be worth a lot. If you don't want to spend more time but have money immediately: pay someone to do it for you, you won't regret it later. Bear in mind that the sooner you find a mistake or invalidate an assumptions made, the cheaper it will be to the right. Thinking about UX properly early on will save you from lots of trouble later on.
