The best marketing strategy first and foremost is always producing good work. If you do an amazing job, word of mouth will spread and you'll have more clients than you can handle fast. After that there are multiple strategies, but my favorite is SEO of course (I do run an SEO agency). :-)
SEO is amazing to implement especially if you do web design since you can link back from each clients site to yours. This is why Web design agencies have notoriously very strong domain authority.
The part most web dev companies get wrong is that they think just by building backlinks will they rank, when in reality your competition is very tough as well, so just like Jasper I'd recommend you focus in on one niche.
Do you specialize in Yoga sites? What about car dealerships? The more you specialize the easier it is to sell clients in that field. Of course if someone outside your niche offers you a project you shouldn't decline, but specializing is amazing especially once you build up a portfolio.
We took on a testosterone clinic as a client for example, did great work for them and then worked with 2 more right after. Now we're building a niche in this field because we know exactly what a great campaign looks like and selling our service is easier.
Agency businesses are commodities, so you have to stand out on things you can control like your customer service, business story and operational process.
Last but not least, try to have at least 3 channels you use to get clients because you never know when one may run dry. 3 I suggest are:
1. Outbound email you can learn through a lot of trail and error, it's a hard game to learn, but worth it once you crack the code.
2. SEO for example takes a while, but can be a great channel that keeps increasing your website traffic monthly and increasing your leads for the same costs.
3. Channel partnerships can be established relatively quickly if you have the right network, nothing beats a great referral from a good partner. Look at companies that can refer you a lot of work (accountants for example) offer them some free website work in exchange for referrals. Meet with your partners once every through months and keep building your other channels as well.
Given how competitive the industry is, I believe it is super important to both specialize and differentiate from everyone else. We faced this same problem 5 years ago when we started The Conversion Wizards.
We had some expertise in paid traffic generation, SEO, Business Intelligence and CRO, and could have tried to become a 'full service agency'. Instead, after sometime banging our heads against the wall we chose to specialize in just CRO and declined some jobs that were not in this core competency, so that we could deepen our expertise and also amass the right kind of case studies. In depth case studies are really important for attracting the really big clients. And sophisticated buyers want specialists. In this day and age, it is very hard to be good at everything.
In your case, perhaps focus on one or two key verticals and go after them relentlessly especially in situations where you are absolutely sure you can add value--usually because you know the industry or market well.
So how do you go after clients relentlessly? Offer to work for free. Get in touch with a short tailored email and say you have reviewed their site and have two specific ideas you believe will improve their results. If you do this enough times you will get some takers. Just make sure you don't work for free too long. We made this mistake and they didn't respect our work.
I'd recommend at most a few hours. And then say "now that you've seen a bit of what we can do, would you consider a low hourly rate. If you let us publish a case study, we can offer you a highly discounted rate."
Actually, I just reviewed your site and it looks like you are much more established than you let on! Maybe show your big clients higher up on the page? Maybe come up with a more differentiated value proposition? "Web design and re-design" specialists is too generic. When I read that, I think to myself, you have an amazing grasp of the familiar :)