If you are fine with chemical treatments, then a pre-emergent herbicide should be applied to the area just before new growth would be expected to form. This is especially important if you have had problems with foxtail already.  A few standard chemical options include dacthal, balan, and pendimethalin.  For an organic pre-emergent herbicide, try corn gluten meal. Sprinkle the product after your new lawn begins to grow to prevent foxtail and other weeds from taking root. Do not use it before growing your new lawn, though, since it could also prevent the desired grass from growing. Apply a pre-emergent herbicide treatment in the spring. If you apply it too early, though, they can lose their effectiveness. For best results, apply the herbicide once the weather warms up and stays warm. Lay it down just before you would expect the fox tail to pop up. The soil conditions need to be ideal for other plants. If other plants can thrive in the soil, they will grow and absorb more nutrients, making it harder for the foxtail to find enough nutrients to survive.  The exact pH your soil should be will vary depending on the type of vegetation you have there. The fertilizer and other soil amendments you use on the area will affect the pH. Note that such amendments generally have a greater impact on the pH if they are worked into the soil instead of spread on top. Herbicides can affect the pH, as well. As a general rule, if you add an acidic herbicide, balance it out with an alkaline substance afterward, and vice versa. Have a sample of your soil analyzed for pH at a lab. When an area of land is crowded by too much vegetation, some of the vegetation will inevitably die off once the roots begin to compete for nutrients and resources.  You could also replace the grass entirely with sod. When choosing vegetation to plant in the area, opt for a solid-stand crop like legumes or grass hay. Row crops, like soybeans and corn, offer too much space in the soil and are not effective at stopping foxtail growth. If you plant other forms of grass in the area, keep the grass as plush, thick, and healthy as possible. Thick grass can also be helpful for use against foxtail growth. Regular mowing can keep the yard healthy, and a healthy yard is less likely to foster the growth of weeds like foxtail. While keeping the area trimmed is important, you should not keep it bare, either. Maintain a minimum grass height between 2 and 3 inches (5 and 7.6 cm).
Try a pre-emergence herbicide. Monitor the pH of the soil. Fill the area with other growth. Keep the area trimmed.