Last Friday SPY had formed a very large 2 bar reversal from a key support and resistance level. This could see prices rotate down towards 2600 before potentially finding some support. As this market has been strongly bullish , caution is more than warranted if you are looking to short it. Although, this looks to be the best bearish setup that I have seen since the initial rally started last December. If price gaps down tomorrow, I may be looking to short once the gap is filled, or even more aggressively waiting for a 50% retracement of the 2-bar reversal pattern.
The weaker ETFs such as XLF or XLE may be even more attractive shorts.