i own over 3000 apps across all apple machines (including adobe software on mac), and many android apps as well. what gets me to buy is this: the app provides me with an eexcellent, fully functional, open ended experience for free. it also lets me know i can have more of the same if i buy. it informs but doesnt nag and doesnt do sneaky self-promotion, and is mostly free of other such nuisances).
if two apps are equally good, or close, the one with the handsomer icon wins. if an icon is really ugly, i will always be searching for its replacement that does the same thing but doesnt make my eyes hurt.
: hipstamatic is a prime example. its filters are so good you want them all. they regularly give new "freepaks" (that revert to paid after a day or so, meaning get them free quickly or buy them at your leisure) and also regularly offers new paid ones. i buy every single one. as do many, many people.
other examples: alien blue, camera+, friendcaster, fruit ninja, talkatone, angry birds, paper by 53, twitterific...notice anything? these are all top apps. users can have a complete and excellent experience without every buying anything. if they want more, they buy. its hard to imagine anyone /not/ wanting more.
teasers -- for me anyway -- are basically ads. as such i tend toward skepticism and if they are frequent, they become increasing irritants. occasionally ive permanently deleted an otherwise good app -- which i might later have upgraded to paid -- from sheer annoyance at being nagged.
this, from someone who eschews adblock. my son programs professionally and i occasionally beta test. i understand that people need to live. but im guessing teasing (unless done really, really skillfully and sparingly) would yield more losses than gains. let me /know how it is to enjoy the app./ let me use it daily until it becomes essential to what i do. then I'm hooked and will certainly upgrade.
while I'm here: dont remove features after someone's paid for your app. or if you do, please read reviews amd if the feature is loudly missed, restore it. i quit using one app because they removed the two reasons i used it: it was the fastest of its kind, and it had a stunning fliter no one else had (i do a lot of iphoneography).
i quit using another app -- that had paid me to help with filters as they were growing -- because they force users to add tags to everything. i don't have time for it. if the image cant speak for itself, then its mute and I'm onto the next one.
if you make a great app, let people actually use it so they /know/ its great, and refrain from being bossy and/or pestering them, you should be able to get many upgrades to paid.