I realized I have been avoiding leaving tips in Foursquare because I didn't have a good way to post them from my website. So today I added the ability to syndicate my posts as Foursquare tips.
I have three different ways my posts can end up as tips on Foursquare:

I can reply to one of my checkins
I can reply to a Foursquare venue URL directly
I can send the text of a checkin itself as a tip

That last one is interesting, because it doesn't quite map to the normal way tips end up on Foursquare. But I realized that sometimes I write text in my checkins that is actually more like a tip anyway, so I wanted to give myself that option.
I'm probably most likely to use the first option of replying to one of my own checkin posts to leave a tip. Since my checkins appear in my reader, I can actually post directly inline from the reader to go back and leave tips.
I also made the decision to not try to syndicate these automatically, instead I have to manually click the syndication option before my site will attempt to syndicate the post.
Here's what a checkin post on my website looks like:

And here's what that tip looks like syndicated to Foursquare:

Overall this was pretty straightforward to do. The hardest part was dealing with finding the Foursquare venue URL from my own internal storage of my posts for the three different options.
The Foursquare API for this is reasonably well documented (Add a tip to a venue), but there were a few gotchas. It's not immediately obvious from that page that you need to include the access token as a post body parameter, and you also need to include an API version string. These are documented elsewhere in their docs, so it didn't take too long to find, but if you just start out reading the specific API method you want to use, it's not mentioned there at all.
There is also a parameter in the API docs called "url" which is described as "A URL related to this tip" with an example value of "http://blog.zagat.com/taco-survey-results-are-here". At first I was excited that I may be able to link my Foursquare tips back to my website. But every time I tried including a URL in the request, I got back a generic error "bad tips/add request". Leaving out that parameter made the API call work. Oh well.
So hopefully this will encourage me to leave more tips on Foursquare! I find the tips very helpful when I travel, so I'm happy to contribute back to this data set!