I doing SEO for a large site (20+ pages). What are the best practices in regards to the sub-pages of the site? Should I put keywords for sub-pages, and if yes how similar can they be to my home page keywords? What about description and page title, how long should those be and how similar to the home page? Is there anything else that can be done for SEO optimization?

seo is not just putting meta data on the data..
It is just basics.

First as you ask how to work on sub pages

if they are commercial pages like services or product related page , then optimize them according to the content and theme of that page..

for ex.
a page www.abc.com/web-design-services.html
then target it with the web design related keyword..

if they are normal pages like team- information, privacy policy or disclaimer

then just make this page seo friendly.

One more thing , if you start with new website, first step to rewrite URL into google friendly. then start work on internal linking, because it will play important role to grow keywords with stable ranking and page rank and third and important , anchor text use to link pages.
I think these are basic to start seo for a site.

I think link building and social bookmarkings is one of the best tactics is optimizing site.

I think you should use effective keywords in your anchor text to optimize your sub pages, the anchor text will work as a short descriptor for your web pages.

Each of the subpages needs to have unique content and tags. Bookmark them to popularize and promote the inner pages.

i am not able to get you..

will you explain please...

Are you talking about the SBM(Social Book marking)

search in google
i think you can get many answers

Make the subpages rank for whatever content those pages contain, keywords you use for subpages shouldn't be different, normal SEO principles is applicable to any webpage, just make sure you optimise the page for that keyword.

This thread is similar to one from a few years ago that just got revived. As I said on that one, I was in a similar situation and what I did was that since each of the subpages dealt with a different product I created a list of keywords to use as both meta tags and within the content of the page. I then created a master keyword list by combining all of the keywords from the subpages and used the master keyword list as the meta tags for the main page. What this did was that if someone searched on a particular keyword the site would place two pages high in the rankings, the home page and the specific subpage. Worked really well on the site I was working on.