What's the best way to ensure that sales uses the sales enablement content that marketing creates...even if there WAS collaboration on the content?

First, create content that’s in line with what you know they will use. I mentioned in one of my other answers here that oy need to understand their selling methodology and ensure the content aligns with it. If they use Demo2Win then know your content needs to follow a Tell-Show-Tell framework around problems, solutions, value for each mini-chapter. If they use ValueSelling, know that your content needs to anchor on a key business issue followed by the common obstacles standing in the way. The more you understand about how their pitch will be run, the better you can build content that will easily fit into it and get used.

Second, ensure everyone actually knows the content and is comfortable with it. Rolling out decks and hoping they get used can be frustrating for everyone. Add speaker notes to the slides, record talk-tracks to the decks that people can review, and run elevator pitch competitions where each person needs to present an overview of the content to their peers and you. Offer up some prizes to make these fun and incentivized.

First, I think you shouldn't expect 100% wholesale usage of the sales enablement materials you create. Sales enablement materials are meant to create a solid foundation for reps to use but, ultimately, every prospect and customer requires some level of customization so expect adaptation. 

There's really a spectrum here of tracking and it depends on what metrics you need to justify the resourcing you put to sales enablement, and to whom you need to justify that resourcing. On one hand, you can simply use anecdotal feedback by talking to reps and sales leadership to understand if the materials are being used. This could come from joining sales meetings and asking, or doing an informal poll. On the other end of the spectrum, you could gate and tag every marketing asset and use technology solutions to track usage. I'm not going to cite specific tools but it won't take long for you to find!

The more tracking you do, the more friction you might introduce into the process and in some ways it might actually run counter to your goal (usage). But if your team needs to justify additional heads for sales enablement, or you need more OpEx to get things done, you could take this approach. 

Regular reminders and reinforcement! Sales teams obviously face a ton of pressure to hit quota, and they need to be as efficient as possible... And while some are awesome at experimentation, many will struggle to adopt new assets/messaging when they're moving fast and rely on what's familiar. 

On the flip side, be sure to manage outdated materials as best you can... If there are old decks/n-pagers floating around, label them as such, and if you see things pop up from people's "private collections" send them a note with the latest and greatest and ask them to replace. 

Make it as easy as possible to use a new asset (discoverable, clear guidance) and then showcase where it has been impactful for other reps. Nothing like an internal case study to motivate your team! 

I don’t just open up a store and wait for people to come. I personally go find the first 2-3 customers that attract more to the store because apparently, it’s the place to go.

No matter what you create, you should build champions for your content within sales. The most effective way to do that is to pick salespeople who are seasoned performers and who others in the team look up to. If you have their stamp of approval, the rest will follow.

Here are a few tips for ensuring your sales team leverages the content you create:

•  Make sure the content can easily be found. Have a central, go-to place that reps can find all your content. This can be a 3rd party platform such as Highspot, Seismic, or Allbound. Or it can be a more simple Airtable/Spreadsheet that has proper filtering if you're on a budget. Of course with a platform you'll get data/insights into what's working, what's not, so I recommend a platform vs spreadsheet.
•  Keep the content up-to-date. If a sales rep loves a piece of collateral, but continues to find that it's outdated and missing the latest info, then they'll quickly lose interest in using it, and they'll lose trust in the marketing team.
•  Host monthly 'marketing/enablement showcases'. At HubSpot, we've hosted monthly meetings that are around 45-60 minutes, and attended by sales reps. During this time, the team walks through the latest content, collateral, and campaigns. And we don't just show what the asset is... we show reps why we created it, when they should use it in the sales process, and often try to share a recent sales-win from using the asset.
•  Showcase any rep wins attributed to your content. This one has worked REALLY well for us. If you find a rep who leveraged an asset from Marketing and it led to a good engagement with a prospect or customer, then get a quote and showcase that to the broader team. Once other reps see another having success, they're more willing to use it. Bonus - if you can show these wins to Sales Managers, who are willing to share with their teams, that'll work even better.

At the end of the day, marketers who support the sales org need to focus on QUALITY and not quantity. Don't get yourself in a place where you're a content factory. Instead, do research with your reps, find what would be most impactful and useful, and commit to a small number of high value assets.