Ways to get affiliate sign up for WordPress themes site.?

There are a number of routes you could go here...
1. Build a blog based around website building and web design. Start posting once a day using on WordPress related subject manner with particular attention to themes. Create a money post targeted with your affiliate link and recommending those themes as the best. If your blog is not already getting traffic, start performing basic SEO and target a specific long tail keyword.
2. Build a list. Set up an autoresponder. This can be by whatever means you like and even with the blog mentioned in #1. If the list is targeted towards web design or building websites is an integral part of the niche, mention the themes sites every now and then in your newsletter.

Those would be my two quickest ways. Another would be to give something away for free and mention your affiliate site in the free something something.

I have implemented the reviews of the themes and provide discount coupon codes and its really help me to get some sales.

Guest Posting on relevant site may be the best way to get more subscriber.

how about submitting in affiliate directories related to wordpress.?

There's one more strategy to get more affiliates. It takes longer than just one day, but it is very good and reliable.
1. You start posting your quality content on wordpress tips, programming, reviewing (where you're good at).
2. Every post, category and home page must be with SEO meta description, title and keywords. Make a good on-site SEO and use long tail keywords. Use pictures with alt tags in your posts. This way you will definitely get traffic from Google. Don't build up backlinks, as they could even hurt your site with recent Panda, Penguin and whatever Google animal updates. Let the links build up naturally.
2. Build up your social presence. Facebook, Google+ and Twitter. Just to get more traffic.
3. Posting in Facebook groups (10.000 - 20.000 members) related to Wordpress. Don't spam, provide good tips and help people. One of my blogs got 100 - 120 views per day just from Facebook groups.
4. As your website traffic starts building up, set up your newsletter subscribe form and some optin plugin. I use Optin revolution pop up. Very good and free with limitations. You can use other plugins as well.
5. If traffic is building up too slowly, start using Facebook ads that will lead to your subscribe page. To get more leads, offer a free product for their email. The target is getting subsribers to your list, as they will be your customers and buyers.
Now that's a very good and instant traffic source, but it costs money. Split testing and all kinds of testing of your ads is A MUST.