Stock market sell-offs aren't fun if you're already fully invested. However, they're opportunities for those who have cash to deploy. That's why I always try to keep some cash on the sidelines -- so I'm ready to capitalize when Wall Street takes a tumble.

I also keep a watch list of stocks that I know I'd like to buy if they ever fall to more attractive levels, and Realty Income (NYSE: O) and W. P. Carey (NYSE: WPC) are currently at the top of that list. These real estate investment trusts (REITs) already offer supercharged dividend yields. However, they'll likely become even more attractive during the next market slump.

A model of consistency

Realty Income's dividend yield at its current share price is over 5%. That's several times higher than the S&P 500's average dividend yield, which is now below 1.4%. This monthly dividend payer has a terrific track record of increasing its payouts: It recently delivered its 108th consecutive quarterly increase and its 127th boost since coming public in 1994.

The diversified REIT owns a portfolio of retail (74.4% of its rent), industrial (14.5%), gaming (3.3%), and other (2.8%) properties, which it rents under triple net lease agreements to high-quality tenants in durable industries. That lease structure makes tenants responsible for all of a property's operating expenses, including routine maintenance, building insurance, and real estate taxes.

Meanwhile, Realty Income's typical tenants are in businesses that are resistant to the impact of both recessions and e-commerce competition, such as grocery chains, convenience stores, and pharmacies. These features supply Realty Income with stable rental income.

The REIT pays out about three-quarters of its stable cash flow to investors via dividends. That gives it a big cushion while also allowing it to retain a meaningful amount of cash with which it can expand its portfolio of income-generating properties.

Realty Income believes it can grow its adjusted funds from operations (FFO) by around 4% to 5% annually. That should support continued growth in its high-yielding dividend.

Back to rising after a reset

W. P. Carey shares a lot of similarities with Realty Income. It's also a diversified REIT that inks triple net lease deals with its tenants. However, it concentrates more on industrial and warehouse properties (64% of its rent), with the balance coming from retail (21%) and other properties (15%). W. P. Carey also has a portfolio of operating self-storage facilities.

The company focuses on owning operationally critical commercial real estate. Because these properties are particularly vital to their tenants, they tend to pay rent on time and renew their leases at market rates. W. P. Carey's leases also typically have built-in rent escalators that either boost rents at a fixed rate or one tied to inflation. Its rents rose at a 2.9% annualized rate in the second quarter, much faster than the roughly 1% annualized rental growth rate Realty Income expects this year.