I just use some Dawn dish soap in a small pump-up spayer with warm water, spray it on, let sit, then hose off. Avoid spraying directly onto/into obvious "trouble" spots--alternator, open element air filters, spark plug area, and CPU connector. I've been doing it this way since I bought my Ranger new in May '04; underhood still looks factory-fresh. I do this every other wash, or while cleaning-up for a show.
2012 Focus Titanium Hatch-6/12/12 Build Date-Tux Black-Charcoal Cloth-DCT-Winter Pkg-Moonroof-K&N Drop-in-Magnaflow 15072
2014 Focus ST2-9/9/13 Build Date-Perf Blue-Charcoal/Blue Recaros-Moonroof-Cobb Intake-Magnaflow 15155-QuickLifts