The most crucial must be
1) have lots of time . For every day in which you plan to fly, you need at least one day spare. Either for unexpected weather or against the case of failure, either of the plane or the pilot. Make the margin larger the further you get from home
Less crucial, yet important:
2) be well informed about airfields near your planned destinations, including their availability of (the right kind of) fuel, and appropriate maintenance/repair facilities
More?
3. Be able to fly VFR on Top.
Willing to fly VFR in marginal VFR conditions / to fly SVFR in control zones. I’ve seen people get frustrated with VFR touring whose expectations regarding weather were just too high and untealistic. There is no way (north of the Alps) that you will have 5 consecutive days of CAVOK in the same general area.
Be flexible and open minded.
I once did a Tour de France which ended up going to Poland. Was a great trip.
good airplane availability
I find living in California helps enormously. 
I find living in California helps enormously.
Amen!
Although I did cancel a couple of trips to Tahoe, and had to do an unscheduled layover (for two nights) in Monterey due to a front coming through the Bay Area with icing. Man was I bummed out I when I arrived in France from California… Took me years of mourning to get over it.
For me, it has been
I have never done a long VFR trip (out of the UK) which was 100% VMC. However I have done many long IFR trips which were 100% VMC! Why is that? Access to airspace is often limited when VFR.
I second Achim’s opinion. You have to be flexible enough, with time and destination.
If you travel VFR, the journey is your destination; you travel to fly. If you fly to get anywhere, then VFR is not for you. If your spouse or acquaintance expects to be in the hotel in time for dinner that has been booked 5 Months in advance, you better have a very capable aircraft (and be a very capable pilot). To go camp-flying in the direction of the best weather, you just pick the nicest cheap VFR aircraft, load your Tablets full of charts and enjoy flying. Most airports where you will strand won’t have IFR approaches anyway…
Last summer on our Venice trip the most important for go /no go decission was the blitzortung.com. This summer may be different.
