Well, some basics if they're any help; keep your spine straight, relax your whole body and keep your shoulders low. Don't open your mouth wide when singing low notes. Visualizing in your mind can often help, especially if you have trouble with your breath support (which could be the reason for your throat straining); take a deep breath and imagine how your voice comes from inside. You can also try going step-by-step, first sing as low as you can when it still feels good and then while trying to keep it natural and not forced go lower just one note at time. That can take time, but when repeated on a daily basis you will probably learn how to pick up lower notes right away without going step-by-step. Hope this helped
It could be that you have reached your limit, but I've heard people claim that with correct training they've widened their range. But like Crystal said before pursuing lower register can take away from the higher so be careful. You could also study a little about singing from internet by yourself (but only professional sites), things like how to warm up correctly and where exactly your voice comes when singing certain notes (throat, head, stomach etc). ^^