1) Strongly, strongly consider capturing CCs at signup. Yes, this takes a bit more work. Yes, this will decrease your conversion rate. However, you're going to get more people successfully charged than you do currently.
2) There exist things you can do to increase user engagement with the app which will increase conversion rate from "user" to "signup", even if you don't take advice #1. I have a video coming out in a little while about this. Short version: go take the free trial of Balsamiq Mockups and take copious notes about what they do. In particular, note how Balsamiq doesn't start you out with a blank screen and note the timing and frequency of prompts for you to signup. Implement both of those things. Your conversion rate will increase.
3) You captured my email address for a "paid" signup. Good! I got no email from you. Bad! You should, bare minimum, hit my inbox with "Thanks for signing up for Codiqa. Your free trial lasts until XX/YY." followed by the best three bullet point sales copy you can come up with, linking back to your site extensively. Why? Because I already forgot your domain name (literally true) and statistically speaking I already forgot I signed up for Codiqa. Hitting my inbox is a free chance at recapturing my attention tomorrow when I do my morning email pass.
4) But wait, if one email is good, isn't five emails better? YES! Put a wee little checkbox on your signup form saying "Can we email you a free course about JQuery mobile? Five emails, absolutely no spam, opt out any time." You'll get 40%+ signups. If they sign up, you hit them on day 0, day 6, day 15, day 20, and day 28 with emails of approximately the following format.
Thanks for signing up for a free month-long course on JQuery Mobile by Codiqa, the jQuery mobile design app you used recently. (You asked for this when you signed up. Don't want it? No problem, click here.) This is your 2nd email out of 5. Today's topic is how jQuery mobile cuts app development costs while increasing revenue.
(h2) jQuery Mobile Cuts Development Costs by 50% (/h2)
Paragraph of copy. (~300 words) Link to codiqa docs or blog in here somewhere.
(h2) Another bullet point (/h2)
Another paragraph.
(h2) Another bullet point (/h2)
Another paragraph
Conclusion paragraph goes here. Early in the email sequence the conclusion paragraph just mentions your service, establishes expectations for the next email, and invites them to email you at any time. Later in the sequence you get salesy as heck.
What does salesy as heck mean? By the fifth email you should, if you're good at writing and jQuery Mobile, have your most engaged customers eating out of your virtual hands. The fifth email is less about education and more about one focused case delivered by their favorite jQuery Mobile expert on why buying the $30 a month plan, specifically, today, is the best possible thing they can do to improve their life today.
Execute correctly on this and you will print money. How much money? Just a comparable: for BCC, which is far less sophisticated on the email than this trick is and which has terrible unit economics, I make about twenty cents per email I send. If I tell you how many orders of magnitude that number can be improved by, four different consulting clients are going to demand my spleen sauteed with a glass of chianti.
5) Charge more! Or at least give an option for charging more. $30 a month is rat droppings to a business doing mobile development. (You will not get that feedback from some HNers. That's OK. Five hundred HNers refusing to buy your product plus one business charged $500 a month = $6,000 of revenue a year.) Put a $250 a month plan in there. You will sell it, to someone who a) does not need to pay $250 of his own money to buy it and b) receives well in excess of that much value from your service.