As of this writing, the S&P 500 index is at an all-time high, having risen 15.6% year to date, 36.6% over the past 12 months, and 57.4% over the past three years. Those are spectacular results, but whenever stocks reach highs like this, many investors begin worrying that a crash is coming. Not wanting to lose their gains, these may be tempted to make major adjustments to their long-term stock portfolios.

In this video from Motley Fool Live, recorded on June 30, Motley Fool contributor Brian Withers talks with contributor Jon Quast and longtime Fool Anand Chokkevalu about what changes -- if any -- they're making to their investing strategies with stocks at all-time highs.

Brian Withers: Well, with stocks at record highs, are you guys doing anything different as an investor? Jon, why don't you take this one first.

Jon Quast: Yeah. To your point, stocks are at an all-time high. What has me thinking hard is that they're also at pretty high valuations if you look back historically. If you track the average of the earnings and the sales for the S&P 500, look back over the five-year average, we're at a very high point right now. It's not just that they are at highs. They're at high valuations, and that really has me rethinking my cash position lately.

I want to have that money. I don't want to invest at the highs, nobody does. But I don't want to stay on the sidelines either. What I've done is, my family, we budget monthly for investments. I'm taking 50% of that money, the new money, and putting it into the stocks that I like right now, whether that's a new position like Etsy or an existing position like Airbnb -- investing that money. Now, if stocks pull back, I'll look to invest all of that new money. If we get an actual crash, then I'll look to take all of the cash off the sidelines. It's just what I've come up with to help my decision-making. There's no magic formula. It's just how I'm thinking about it right now.

Anand Chokkavelu: Great. Usually, it's probably somewhat similar. It's kind of the usual for me where you get lots of periods of hand-wringing, and poring over my stocks, and wondering, and thinking, and doing that rank-order exercise. This is where conviction helps. But not much in terms of actual action. Lots of talking with fellow investors, lots of hand-wringing. On the margins at this point, if I get a new chunk of money -- say a company I have gets bought or where I sell something -- I might push for more value in international names to balance my growth- and U.S.-centric portfolio. I'll probably rebuild some of the dry powder I exhausted completely last year for the most part. But overall, staying the course, buying great companies with great futures, and holding for the long term. Just little things at the margin that makes you feel like you have some action when you're not really doing much.

Withers: [laughs] I love how you explain: "It's the usual, lots of periods of hand-wringing." [laughs] But I'm very similar to you. If you buy great companies and really prioritize -- I only have 19 different individual stocks, so for me, I've really prioritized down to my best ideas. I'm pretty much fully invested now, and I pretty much have been fully invested my entire career since I started with The Motley Fool in 2004.

As new money comes in, when I was working, I would put that money to work. Now that I'm in a semi-retired state, I'm still fully invested. I have cash, an emergency fund and whatnot on the sidelines, so that if life happens, we don't have to sell stocks. But for me, right now, if I want to buy something, I have to sell something else. With only 19 stocks as I've really prioritized down, that's really hard to make that extra choice. It's a little bit of extra friction that helps me not to mess with my portfolio all the time. I do look for opportunities to sell, but it's somewhat tax-year dependent. I've racked up enough taxes this year, so [laughs] not planning on selling anytime soon. That's my process and what I'm thinking today.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.