The Lake Allen "Trail" is an old fishermans' trail that, as of 2007, is quite visible and even designated with orange markers in places. This makes getting to Lake Allen and subsequently the scramble routes up Mount Wow quite convenient. In no way is this an "easy" route though, the trail gains roughly 2000 feet in elevation in a single mile to reach the lake. (ouch!)
To reach the trail enter Mt Rainier National Park and turn off on the West Side Road. Follow the West Side Road about 2.3 miles, being on the lookout for two significant drainages on the hillside to your left. You will pass one, then go a short distance and pass another, then there will be a pullout area where you can park. From the car, look for another drainage that is likely to be dry for most of the summer. Enter the woods and cut across the slope going to your right (North). You should find the trail fairly quickly.
The abadoned trail (see attached historical topo map) follows a step but logical path most of the way up to Lake Allen. Once the trail starts to flatten out the trail heads to the left (West) to cut back towards the lake. You will go down a short distance, cross two small creeks, then head up a hill at which point you can see the lake. From here, you can follow any of the already documented scramble routes to reach the summit of Mount Wow.
Descending the Lake Allen trail is tough but quite doable, make sure to bring a trekking pole or be comfortable using your ice axe for balance. Those with knee problems be forewarned, the descent will not be fun for you.
Ice Axe, Trekking Pole, Topo Map, Compass, GPS (optional but highly recommended).