I always begin with the apology:

"I'm terribly sorry, but I've had to land my glider in your field." (though note I'm from the UK, where "Sorry" is the proper response to someone treading on your foot).

This makes two points: I know I'm being a nuisance, but I really had no choice.

Then I usually say something like "I don't think I've caused much/any damage, but you'll want to take a look." Once it's obvious that I've not destroyed the field, the conversation about removing the aircraft is usually very pleasant, and I've been lent tow vehicles/given a hand/etc.

It helps if you know enough about farming to show that you won't cause further damage, e.g. that you'll drive only on the tractor wheelings so as not to compact the soil, and so on.

Things can be different at ridge sites, where there are often known safe landing fields and a standard fee because they get used many times a year - the local club briefs visitors on those here.

All the other advice is good, but I've found that starting with the acknowledgment that you're an unwanted visitor is what's needed to form a good relationship.