People have thrown the phrase "personal minimums" around for a long time, but I think there's been something missing from the discussion. And while I am writing this for Sport Pilots, it really applies to every pilot.
The safety gurus try to make the "personal minimum" decision sound easy, but I've never found it that clear cut. A few years ago I was flying some very nicely equipped King Airs, took annual simulator training, and had to take a semi-annual instrument check. If it was legal I could handle it. Well, at least "legal" in the King Air. One winter afternoon I took off in my Taylorcraft and headed west across the desert. It was VFR but the white sky and snowy ground meant that there was no visible horizon. The T-Cart had a venturi driven turn needle that seemed to work best on hot days, so my instrument proficiency was of absolutely no use. I turned around and landed.
In other words, a pilot who was IFR proficient below 1/2 mile visibility cancelled a flight because it wasn't good enough VFR. It takes two to tango.
The FARs contain a huge loophole for charter pilots (14CFR135) that I flew through regularly. To launch on a charter flight you could use any combination of reports and forecasts to make sure that the weather would be legal when you arrived. So a forecast for bad weather that was worse than the reported weather meant I could go. Incredibly poor reported weather could be ignored if the forecast said it might improve, even if every ounce of common sense indicated that the forecast was wrong.
FAR 61.315 spells out sport pilot privileges and limitations. These seem pretty clear-cut: no night flight, no flight above 10,000 feet MSL except within 2,000 feet of the surface, no flight with less than 3 miles flight or surface visibility. The last one means no Special VFR, and no taking "advantage" of the reduced visibility required by FAR91.155 for Day VFR in Class G airspace. (This so-called advantage is the classic example of something that is legal but not safe.)
Furthermore, a Sport Pilot must maintain ground contact. A Private Pilot may legally fly above an overcast, and sometimes this can be safe. Sometimes. But a Sport Pilot may never do this.
The reason Sport Pilots have these limitations is that they are not required to have instrument training, while Private Pilots must have three hours of "hood" time. It always seems a shame to put someone under the hood on a pretty day, but still I make my Private Pilot students fly to the practice area and back under the hood, and they also spend some cross-country time unable to watch our lovely scenery go by.
Even if you are a private pilot, you are limited to these conditions if you have limited yourself to Sport Pilot privileges by using your driver's license as your medical.
So what's the problem?
The problem is that there's no mention of the forecasts.
I've visited a few EAA chapters recently and I have heard a lot of talk about Oshkosh. Some of it has come from me, because the last time I made it to Oshkosh was before my teenagers were born, and it's long past time to go again. As I think fondly of the great people I met there and the wonderful things I saw and learned, I can't help but recall that every time I have gone to Oshkosh it's been IFR. I think one time the visibility was 2 miles, but never 3 while I was flying. If you get a standard briefing then you'll get a NOTAM for IFR conditions. Pay attention.
Enroute it may not have been IFR but VFR flight was a lot more comfortable above the convective layer. If there are scattered cumulus then it's OK for a Sport Pilot to be "on top", but when the clouds are broken being on top is questionable and when the clouds are overcast it's illegal. The Area Forecast should give you some information about the cloud tops.
Any trip east from my home airport involves lots of mountain flying. There is a route from home to Oshkosh that stays below 10,000 feet MSL, but there are lots of wind conditions that make 2,000 feet above the ground too low for my taste: think about the Steve Fossett accident. And more than once I've found the valley I intended to fly through blocked by rain or snow. "Sometimes you just have to fly toward the blue sky," but what if that means climbing above 10,000 MSL? Can your airplane do it? Can you do it, legally?
So you really have to pay attention when you see those little hints like VCSH in the TAF. (That means showers in the vicinity.)
The missing piece of the personal minimum decision comes from understanding the forecasts. TV weather is fine for a local flight, but when you set off on that looooong cross-country you really need to understand the forecasts.
Your ability to obtain, read, and interpret a forecast becomes part of your "personal minimums." If you can't understand the forecast then you shouldn't go.
Any pilot who intends to fly any serious cross-country really needs to practice with TAFs and Area Forecasts