There is no 1 thing you can do to ensure delivery, however one thing which could well help you is to send your emails to another server which uses spamassassin (or similar) which documents how it tags your email, and make appropriate adjustments. [ Experience shows that a lot of players who have "big servers" either use Spam Assassin in the background or an equivalent engine which looks for similar things.
Another solution you can use is to "make it someone elses problem", and relay outgoing email through your Internet provider.
Also, particularly if you are just starting out, you will find that some providers will limit the volume of emails you can send in a limited period. Luckily these limits tend to get larger as they see legit traffic from you, but it takes time. (Sometimes the solution here is to send email directly, then if it fails relay through your ISP - either that, or wait as typically they don't stop you sending, they just delay you and make you retry).
Also, set a reasonably aggressive initial retrys (60 seconds, then 5 minutes), which can help deliver emails faster if the provider is doing greylisting.
Also, check your logs - they should provide information as to whether the provider is accepting and discarding your email, or not accepting your emails at all.