When I do math in my head, I usually combine a few approaches:

 * Always keep track of units.
 * Try to re-use math I have memorized (such as the multiplication tables, powers of 2, powers of 3, common square roots, common trig ratios, *et cetera*)
 * Use "carrying" to split each number into a round number and an "error" term.  Often, the first operations will be on the number(s) that are closest to round numbers.
 * An "error term" can either be a number (such as 2 or -4 or 0.216), or a percentage of the round number.  For example, 0.3048 is 1.6% more than 0.3.  In the example below, I convert "0.3048" to "0.3 (+ 1.6%)".  A more standard way of writing this would be "0.3 * (1 + 1.6%)".
 * Use memorized values to handle round number inputs to square roots or trig functions, and interpolation to adjust for the "error" terms.
 * Chunk four-digit numbers into pairs of digits.
 * Use my fingers to store values as Roman numerals.  (This allows storing a pair of digits on my hands.)

For example:

 * 99 * 58 = (100 - 1) * 58 = 5800 - 58 = 5700 + 100 - 58 = 5742
 * 2048 + 1296 = 2048 + 1300 - 4 = 2044 + 1300 = 3344
 * 506 + 998 = 506 + 1000 - 2 = 504 + 1000 = 1504

Since the original poster asked (in a comment) about square roots:

$$ \sqrt{\frac{2 \cdot 0.3048 \text{ meters}}{(9.80665 \frac{\text{meters}}{\text{second}^2})}}  $$
$$= \sqrt{\frac{2 \cdot 0.3048 \text{ second}^2}{9.80665}} $$
$$= 1 \text{ second} \sqrt{\frac{2 \cdot 0.3048}{9.80665}} $$
$$\approx 1 \text{ second} \sqrt{\frac{2 \cdot 0.3 (+ 1.6\%)}{10 (- 2\%)}} $$
$$\approx 1 \text{ second} \sqrt{\frac{0.6}{10} (+1.6\% +2\%)}$$
$$= 1 \text{ second} \sqrt{0.06 (+1.6\% +2\%)}$$
$$= 0.1 \text{ second} \sqrt{6 (+3.6\%)}$$
$$= 0.1 \text{ second} \sqrt{6.216}$$
$$= 0.01 \text{ second} \sqrt{621.6}$$
$$\approx 0.01 \text{ second}\cdot (\sqrt{625}-\frac{3.4}{625-576})$$
$$\approx 0.01 \text{ second}\cdot (25-\frac{3.4}{50})$$
$$= 0.01 \text{ second}\cdot (25-\frac{6.8}{100})$$
$$\approx 0.01 \text{ second}\cdot (25-0.07)$$
$$= 0.01 \text{ second}\cdot 24.93$$
$$= 0.2493 \text{ seconds}.  $$
(Actual value is 0.249322 seconds, at 45 degrees latitude on Earth.)