Landing page - where the conversion happens. Generally a landing page consist of a form that have a few fields to enter, asking your customer to complete an action. Either register an event, subscribe to newsletter, buy now etc.
To make conversion better, make your ad money more effective, these are some advise to take:
Call-to-action is the soul. Most of the time it is a form, and asking user to fill it out and press the submit button.
1. Form title
"Fill the form to download the ebook", "Sign up to get a free product sample". They should be clearly state the value of filling the form.
2. Submit button text
"Submit" is good, but in some case there are better ones. "Subscribe", "Download now", "Apply now" are better in some case. General rule: use action, verbs.
3. Form length
Only have needed fields. Longer the form, worse conversion rate. Even optional fields reduce conversion rates as they make the form looks more complex. Go for simple, absolutely needed fields only.
A Good headline
A Good headline make a huge difference. This is also the most A/B tested element. Easily make 10% difference between different headlines. How to write a good headline is out of this post's context.
Images, key visual
A good image is better than a thousand words. Get a photographer to take product shots. Get your designer to pick the best product shot, the best editorial shot and start from there.
It will be even better if your ads, the landing page and product detail page use the same shot but in slightly variations(A/B test). In that way, customers feels more familiar to the visual, but will not feel board about it.
Don't be greedy on content
Keep it short, but cover important things, like a skirt
This rules also applies here. Laser focus on one thing to tell, one thing to teach your customers, one product information, one form to fill. Keep it short and neat.
You may cross sell related product/service, but please put them in the very bottom of the page, or even better, in the thank you page after call-to-action completes.
Keep it improving
Doing it the digital way. Log traffics, conversions, source/medium, A/B test them to the death. Only applies if the landing page is not time-sensitive (not for a physical event with dates)
Image by @Kevin Gill