Make your content interesting
Not every user is going to want to immediately convert on arrival to your site. Make your content interesting, relevant to the terms that brought them there and tightly coupled to the goal achieved by completing the form. It's reasonable to assume that someone who has searched for something, found a well written, interesting article on the subject and hung around to read is interested enough to convert without a popup. Content is king.
Reward users for completing the popup
Loads of e-commerce sites use sign-up popups and they immediately reward the user with a signup discount. Be very clear about this so the user can comprehend in a split second what they get and make it good too (hard discount is better than entry into a prize draw, for example)
Accept it if they close it and use the rest of your site to get them to convert
Don't make it hard to say 'no thanks' or close the popup and don't make it re-appear repeatedly. Instead observe point one and should a user ignore the popup then rely on strong content to convert them.
Try not to interrupt content consumption
Because if you do the popup will just be closed because the user will want to continue consuming the content. Time the popup, so immediately on the second page view for example. This is a very tricky thing to work out
Put big links to your form clearly on the pages
If your site is all about filling a particular form in then the content will be relevant to the goal achieved by filling in that form. Make sure the user can get to the form easily when the content has got them to a stage where they may consider converting
A / B Test
You can always split test to see which what pop-up timing or design converts the most users and the quality of the resulting conversions. To split test you divide visitors up based on some (possibly arbitrary) metric and display the different sets different things to see what causes the most interest. This can be a simple A/B test, or a multi-variant test This way you can use real data to decide exactly when and how to show the popup in order to attract users who are actually interested.