1. Give plenty of notice
There are a lot of wedding traditions from the USA that are probably best left to our transatlantic counterparts, but the Save the Date is not one of them. Asking people to mark your wedding date in their calendar as early as possible reduces the possibility that you'll be missing a key guest due to prior commitments. If you know the date and you've booked the venue and the wedding is within the next 12 months, don't hold out on sending those Save the Dates. Nobody will complain about advance notice!
2. Consider how they'll travel to and from the venue
It's wonderful that you've found the perfect rural church or hidden-away country house for your nuptials, but consider your guests' travel bill. Of course it's your prerogative where you marry, but choosing a difficult location with limited transport options and a lack of affordable accommodation puts a huge strain on your guests. Think about the kind of people you're inviting. If they mostly have cars and some level disposable income, great. But if all your friends are urban 20-somethings who use public transport, asking them to spend a weekend in a £200-a-night country house hotel a four-hour journey from home is big ask. If you have your heart set on somewhere that's tricky to get to, make sure you provide information on affordable accommodation, and consider putting on transport where you can (at least between the church and reception venue and possibly to the nearest town at the end of the night).