Can you suggest ideas on increasing the value of a domain, while it is idle, either because is being offered for sale or because it is still under construction, your hints on SEO, keywords, directories etc. will be so much appreciated.
I'm sorry to say but the very best tip for increasing the value of a domain is to choose them very carefully, before registering them.
Developing a website doesn't really increase the value of a domain. It lends value but that's the value of everything external to the domain name itself, especially those externalities that have some degree of permanence.
You want hints on SEO, keywords, directories? Read WebmasterWorld. Extensively.
There's no fast and easy way to online success, unless it's the fast and easy way that marketers of "success" products/courses make their money - peddling Webdev basics to the uninformed, the lazy, the greedy - and taking the fool's money in the process.