There are a few types of tools to consider for conversion optimization.
2. AB Testing - these are for execution of your experiments. Google Website Optimizer, Optimizely, and others can help with website AB tests, but you might want to AB test your emails (newsletters and transactional).
3. Engagement - these can help you assist your prospects in the purchase funnel, present users with a special offer, message users with tips, etc. I like WebEngage suite of tools, but have seen a lot of sites use Olark.
The main challenge for conversion optimization activities is 1) having sufficient numbers to achieve statistical significance 2) positive ROI to justify efforts. This is especially true for properly implemented AB testing, so I would advise startups to leave it for last unless your volume of traffic and business justifies the efforts.
For startups pre product/market I would advise to use analytics tools and make simple/obvious changes first - value proposition statement, call to action links/buttons, pricing and offers. Just make sure to use comparable conditions - say 2 week time cycle - and be mindful of external factors (referring traffic, seasonality of business, etc). Engagement widgets seem to be the easiest to setup and can be used for testing offers and/or to collect data on abandonment, any confusion, or customer emails for follow up.