How to get more relevant ads

The ads I get from google have almost nothing do to with my site. Of course very few click those adds.
Is there anything I can do to get more relevant ads?

if you mean google adsense, the only way to get relevant ads is to optimize your keywords in your text...

My ad system is built on JSP includes. The site is including ad.jsp in the site. Will this affect which adds I'll get?

I also show my adds on pages which you have to log on to see. Will this have any consequence for the relevance off the adds?

Is it possible to add good targeting keywords in comments to get better ads?

You want advertisements to be relevant to the body copy so the ads are useful to your viewers and they can hopefully buy product from affiliates.

In the case of google adwords use these techniques.

All five must MATCH:

1. Rev text in TITLE
2. Rev text in H1 TAG
3. Rev text in ALT TAG
4. Rev text in ANCHOR TEXT
5. Rev text in BODY COPY

I asked google about pages with login in. The said that I shouldn't put adsense code on those pages because I would get none relevant adds.

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Actually,, no
Domain and title are the biggest influence.
Ironic that for contentual advertising the content means very little in the way it displays.

I agree with Koalaseo when he includes the Domain between the elements that are the biggest influence.

I have a domain that contain the world travel, even for pages that have little to no content related to travel (not in title, or h1 ect), it shows travel related ads.

Thats because i know my shit dude

Have you tried adding relevant keywords or content that you would like to target? Because, the ads are thrown up based on the keywords, text found on the page. There may be special cases where Google is committed to serve certain ads irrespective of the target site content.