A lifetime isn't long enough to experience all Western Europe has to offer: prioritise what you most want to see, plan realistically and have back-up options in case of unforeseen circumstances such as adverse weather.
Western Europe gets very crowded in the warmer months; reduce time spent in queues by booking tickets to top attractions in advance wherever possible.
Check whether festivals and events are taking place before you arrive; these can be a great way to experience local culture but accommodation can be impossible to find or astronomically priced, and shops and services may be closed.
Lingering at a cafe terrace, pub, park, market and other places locals hang out is as much a part of experiencing Western Europe as ticking off the major sights.
Keep your electrical charger (and adapter if needed) in your day pack.