What are some strategies for keeping solo travel costs down?

Many hostels / hotels offer single occupancy rooms so if you really want to stay in a private room, it's good to take some time to look up ones which do. CouchSurfing is an option, AirBnB is too although I've never used the latter. Charging for two people is the norm when booking accommodation, and I usually don't bother bargaining for lower prices. The "use less electricity" is on dodgy grounds anyway as it's probably not a big component of the cost of a room; it's about the lost opportunity to sell the bed to two people.

Another cost that adds up quickly when travelling is transportation. I mostly travel by public transportation but then there are certain journeys that cannot be made within a city without a taxi or inter-city travel that must be done by cars. Here, staying in a hostel helps as you can often share travel itineraries with other travellers and pool in together for a ride. It's more fun to share the experience, and it reduces costs. The easiest rides to share are ones from airports / train / bus stations as someone or the other does have to head the same way as you.

Joining up with other travellers also helps for joining tours, as many tours often have a minimum number of people needed to book one. Here's where you can bargain hard - especially when you've a large group and going to bar (this is useful if you have a group and happy hours are over - you can still often get discounts on your bill if you ask sweetly; make sure you sort this out before ordering), booking an adventure tour, etc - if you've a group of 3-4 people and above. All this usually works in Asia - your mileage may vary elsewhere in the world.

The 'problem', really, is often just overcoming the mental block of staying in a dorm or approaching a stranger to 'team up' for travelling. It doesn't always turn out to be a pleasant but it's an experience nevertheless. More often than not though, it is more fun, you get stories to tell, and helps reduce costs.