Hello Anton,
The website looks great! It's good that you're ranking good for those keywords.
This appears to be your brand name though. I suppose to want to rank for terms without your brand query in it.
I can see you have blog posts. I can't find a page with all of your blog posts though. I can only see the last four blog posts at the bottom of the page. Perhaps you could consider adding a link to a blog hub page.
Staying on top would be achieved by creating awesome content for your blog, having a good social presence and getting a lot of links and social signals. Since you have a nice portfolio and good blog posts these links will appear naturally over time. You can however speed up the process by reaching out to people who could link to you. Think about bloggers and journalists in you area who write articles on web design bureaus or local businesses.
Press releases are rather outdated if i'm not mistaking. I am not sure what you mean about the guest blog posts.
Do you mean summerizing several blog posts into one post and posting that on someone else's blog?
I would not recommend this since the content would no longer be unique. Check out this amazing article on guest blogging right here from Moz.
Tips for staying on top - You seem to have good testimonials. Do you have a Google local page? The testimonials could also be inserted here by your clients. You could still place their reviews on your site manually after this.
- When you finish a website for a client encourage them to spread out the word about the new website through the media and make sure the resulting articles have a link or brand mention to you.
- Keep blogging about relevant topics which add value to your users.
- Consider creating content which could easily go viral such as infographics and awesome videos. This could complement your blog posts in a good way.
- Make it easy for users to find you on social media channels. At this moment the buttons in your footer don't actually link to your social media profiles. They seem to just refresh the page.
- Make sure your website doesn't contain crawl-issues by using Google Webmaster Tools
- Validate your HTML. At the moment not all your pages are valid -> W3C validator test
Hope i was of assistance, if you have anymore questions please let me know.