While it helps to have tact when upselling to guests in person, there’s no reason to make your menu options subtle. Make guests aware of all the upsell options available by writing it directly on the menu, and train your staff to call these out in conversation.
If it’s possible to add bacon to a burger, include “Bacon $2” under the burger menu item. If your guests want to add chili to their nachos, put it right there on the menu, and you’ll get them thinking about how delicious that app would be with a little something extra.
This also allows you to get more creative with your menu without feeling like you have to offer expensive items for every dish you send out. To automatically include bacon, a fried egg, and fresh jalapeño slices on a burger would cause that item to be pretty pricey, but giving your customers the ability to mix and match ingredients to create their meal gives them a sense of ownership and saves you from the common complaint of sky-high prices.