Resisted Sprints are excellent for improving speed, and the SkLZ Speedsac is a great tool for them. Affordable (less than $50), easy to use and easy to store, the Speed Sac can be used for a variety of drills, including:
The Speedsac consists of a yellow canvas/black plastic bag wide enough and strong enough to hold up to a 100-pound plate, so it's quite versatile in terms of workload. An adjustable belt, fitting up to a 48-inch waist, connects to a strap that is also adjustable. The sac has three canvas pouches for holding weights, each big enough to hold sand, gravel or 10-15 pound dumbbells.
Another feature of the Speedsac is its design. The canvas and hard plastic bottom makes it field- and turf-friendly, so it can be dragged without harming a practice or game field. It also works well on indoor courts and tile surfaces. It is durable enough to accommodate both indoor and outdoor training. The only negative is that it is hard to drag on concrete or asphalt surfaces.