How to promote my design sites?

You should do some article marketing - buy/write articles based on your niche and add them to article directories.

Try getting some good backlinks, affiliates are also a great way to market

Hope this helps

Cheers,
H.

You could also try to do some SEO on your site.Target some keywords and try to rank for them.It takes work to do so but it totally worth the shot.

SEO is one of the best way to promote you website organically. In the initial stage perform some On Page optimization and then continue with Off Page submission which mainly involves of submission to various directories, search engines and article websites. Discussing in social networking websites too plays a great role in driving some some to a website.

In addition to SEO and other suggestions above, I believe an affiliate program that is well managed is a good idea, especially for the logo service. I take it your customers can order and pay off your site? If so, an affiliate program is perfect because affiliate marketers are assured the sales will be tracked as opposed to emailing you and/or calling you in which case affiliate tracking is not good. Affiliates want to be assured their referred sales will be tracked and they'll be paid.

That said, you'll need to attract affiliate marketers. It sounds like your logo service is for small businesses and freelancers. Perhaps even bloggers. Look for affiliate marketers that publish blogs in the small business niche.

One other suggestion is placing banner ads on high-traffic sites to gain exposure and branding. This costs money up front, but can get you traffic ASAP.

Facebook Pages are growing in popularity, so I would write a series of posts on your site targeting the existing search terms for Facebook Pages, especially for businesses. There's plenty of relatively low-competition keywords (I know because I just published a couple of articles about it and did some kw research).

I suspect these keywords will become more competitive in the not-so-distant-future due to the increasing demand for FB pages. If you get decent rankings in the SERPs, you'll be established, which is always better than clawing your way to the top of the SERPs. Just keep up the SEO on the FB pages articles on your site that rank well to retain your ranking.