I never thought sweaty armpit stains would afflict me until recently when I wore a long-sleeve top at a crowded bar. It was definitely hot in there and no Nelly, taking off all my clothes was not an option!
Had I bought myself these Fashion First Aid Stopits Underarm Garment Protectors ($20) I could have avoided that problem. Simply apply the stick-on cotton liner to your clothing and you're dry to go. Available online at Asos.