Most important -------> Pro tip: people who are repeat backers on crowdfunding usually back projects in multiple categories so doing a cross-promo with a project outside of your target audience should not be a deterrent.
You can even see this in action. Go to any backer profile and click into their profile. If their profile is public, you can see which of the many Kickstarter categories your backer has backed before. More often than not (really), you'll see that the backer has contributed to campaigns across the board from film to design to games to food.
There are two ways to get these cross-promos:
1. Campaigners that reach out to you
Throughout your crowdfunding campaign, there will be other campaigners who reach out to you. When deciding on which campaigns to conduct mutual cross-promotions with, make sure to adequately vet the campaign so that you are not accidentally promoting a direct or indirect competitor to your own product.
For example, if you’re launching a new phone case that makes a cat noise every time you pick it up (hypothetical, but why not, right?), be sure to entertain cold emails from campaigners who are crowdfunding for an e-bike since Superbacker Andy can potentially be an ebike enthusiast who loves cats.
2. Campaigns that you reach out to
On the flip side, be sure to constantly be looking through crowdfunding platforms for exciting campaigns to cross-promote with. It can be campaigns that have the same target demographic as your own campaign, it can be campaigns with complementary products and so on. MeowCaseTM mentioned above should be interested in cross-promoting with other campaigns in the phone accessories category.
One concern many campaigners have is the price point for campaigns they conduct cross promotions with. What if rewards in my campaign are $1,000 but the other project rewards are tiered at $29 and $39?
My advice to you? No cross-promo is a bad cross-promo.
Again, repeat backers look at different products and categories and vet them accordingly to their competitors within the product category. Even if your product is $199, be sure to reach out to campaigns at higher and lower price points too.
If your product is appropriately priced within the market then this will not be much of a worry.
In case you're looking for what to write, check out this article here:
https://crushcrowdfunding.com/3-killer-examples-of-cross-promotions/