Most good tennis players run into a problem as their skill increases that involves finding good practice partners. I have solved this problem for myself through a number of tried and true methods such as wall practice which is a staple of any strong player as they improve. Here is another approach I have taken to over the past few years as scheduling and finding partners has become even more difficult.
Backhands Down the Line Past a Net Player
Take your basket to the practice court and instead of just serving I suggest you begin tossing balls a few steps away so that you can practice set up for targeting shots. For instance, if you want to practice backhand down the line shots begin from the center mark of the court and for a right hander just toss the ball up and to the left while working for precision on targeting a down the line shot going past an imaginary net player. A passing shot doesn't need as much speed as a down the line used to punish a baseline positioned opponent so I always go for minimal net clearance and very tight tolerance to the sideline. If you are a one handed backhand player you can hold four or five balls and keep moving yourself further to the left as you hit a stream of passing shots past your imaginary opponent. Keep in mind that depth is not quite as essential as low clearance and tightness to the line for this.
Backhands Down the Line Past a Baseline Player
Usually this shot is done as a follow up to a wide crosscourt or inside out to your opponents left side of the court or their backhand assuming they are right handed. After they have been pulled far out of position your job is to find the down the line shot deep with speed to complete the point.
So what you need to do is replicate the previous passing shot drill but instead of hitting with lots of topspin to force a low and short ball you will flatten this one one a bit to gain depth and relative speed compared to the net pass. Keep track of your results as you go and work toward getting excellent parallel travel to the sideline.
Targeting methods and Techniques
For most one handed backhands the safest way to achieve a straight down the line shot is a lift backhand that simply involves an arm lift with the right shoulder pointed directly down the line. Experiment and confirm this for yourself but I can tell you that it is tried and true as a method of straight passes with minimal drift off the line unlike a rolled backhand that ends up generally pulling crosscourt. If you want to see a good lift backhand example check out Lendl. Watch as he hits his trademark lift backhand.
How can you differentiate between the lift and roll? Notice on his finish that his racket face is still somewhat going on the direction of the shot versus a roll backhand that finishes on edge like this Mac backhand, notice the third shot Mac hits in this point which is a roll backhand, sorta. Ignore the chip backhand, the second shot, for our purpose here as well as his first shot the forehand. You will see with Mac his finish actually comes across his body and the racket is in edge with the face of the racket pointing to the sidelines rather than the target. This is a powerful shot with lots of spin but is more difficult to perfect for a down the line shot.
For the Two handers my general rule is to aim your right shoulder to the line and step forward with your right foot toward the target allowing a full weight transfer from the rear foot as you strike the ball. Think of this as a left handed forehand to get the feel of it and make sure at completion that you are facing the target where before you began the stroke you were sideways to the net.
Adding Speed
To generate speed on your shots is a timing skill and to do this with a ball that sits rather than comes at you requires superb timing and judgement. The drop and hit method of practice will improve your ability to add speed to shots especially those that have minimal forward speed.
How to practice and how long
My normal recommendation is to alternate between hitting some serves and drop and hit groundstrokes. Assuming you have a 75 ball hopper with maybe 70 balls in there I would at least go through 3 baskets to justify calling it a training session. This should take about 30-45 minutes depending on your pace of activity.
Measure your results
See how many balls collect on your side as the first means of judging. Next look at where the shots land and the path of travel and be honest about your effectiveness. Over time this will pay dividends in play as you win points by using well practiced shots.
