Some general tips regardless of the border you are crossing:

• in places with multiple individual lines, when choosing which line to join, don't just look at the length. A line of 30 people who form ten families will go quicker than a line of 30 individuals. With experience, you may be able to spot the person who will have a longer conversation with the officer, and avoid their line.
• do not lie. Ever, even about something tiny. They can tell. It may not result in charges or fines, just in getting searched, which is a delay.
• be consistent. If you are here on vacation to see your sister, why are you wearing a suit and carrying a briefcase? If you're here for a wedding, why do you have a hiking backpack with a sleeping bag etc attached to it? If you don't look the part of what you've written on the form, they'll investigate. Again, not saying they will fine you or refuse you, but they'll investigate.
• don't be too slick. Answer what you're asked, and try not to have prepared sentences ready. If they think you've rehearsed a cover story, ... well, you know.
• declare everything. Most stuff is allowed in.
• when you're coming home, you're right that they have to let you in no matter what. But it doesn't have to be fast. So all of this still applies coming home.

For the sake of those behind you let me add: be prepared. Have your form filled out, your passport in your hand, and so on. This can all go nice and quick if we all do the right thing. When you're held up, everyone behind you is held up too.

Some general tips regardless of the border you are crossing:

• in places with multiple individual lines, when choosing which line to join, don't just look at the length. A line of 30 people who form ten families will go quicker than a line of 30 individuals. With experience, you may be able to spot the person who will have a longer conversation with the officer, and avoid their line.
• do not lie. Ever, even about something tiny. They can tell. It may not result in charges or fines, just in getting searched, which is a delay.
• be consistent. If you are here on vacation to see your sister, why are you wearing a suit and carrying a briefcase? If you're here for a wedding, why do you have a hiking backpack with a sleeping bag etc attached to it? If you don't look the part of what you've written on the form, they'll investigate. Again, not saying they will fine you or refuse you, but they'll investigate.
• don't be too slick. Answer what you're asked, and try not to have prepared sentences ready. If they think you've rehearsed a cover story, ... well, you know.
• declare everything. Most stuff is allowed in. I write clues on the form. For example on my most recent trip I checked yes to food and wrote WINE, CANDY next to it. I learned this after seeing what they do when you check yes and they ask you about it.
• when you're coming home, you're right that they have to let you in no matter what. But it doesn't have to be fast. So all of this still applies coming home.

For the sake of those behind you let me add: be prepared. Have your form filled out, your passport in your hand, and so on. This can all go nice and quick if we all do the right thing. When you're held up, everyone behind you is held up too.