The first thing I'd recommend is to divide the inputs into digestible chunks. Make sure you're not presenting all of the open fields on one form. Bring the user's focus to one section at a time. For example, use gentle highlights or outlines on the first section, and disable or even hide the next sections.
The second things I'd recommend is to acknowledge the user's path in completion, using check marks or green cues as each sections is completed. These mini "finish lines" help keep the user engaged and encouraged to complete the form.
Finally, scrutinize heavily what information is absolutely critical vs what isn't. For example, if the user is entering a credit card number, and they have to select whether it's a Visa or MasterCard, that's redundant. The first number of the credit card dictates that, so you could have the system be smarter by recognizing that.
Lastly, if you have a huge number of sections, don't number them...and let the user save where they left off to come back later. Depending on your user testing, that may be necessary if it's an exhausting process. Good luck!