Crystal PCB Layout Design on Re spin Board
First, I've to warn you about the clock frequency. I don't know your desired baud rate, but 8MHz is not acceptable for high-speed (e.g. 115200bps) proper UART. Have you checked the datasheet? UBBR n register determines the communication speed with the following formula (p.227):
$$
UBBR = \frac{f_{OSC}}{16 \cdot Baud} - 1
$$

UBBR must be an integer between 0-4095, Baud is an integer multiples of, say, 2400. So, it's easy to calculate that oscillator frequency should have a frequency of \$f_{OSC} = k \cdot 1.8432 MHz \$ (k is an integer). Example: For 115.2kBaud, UBBR is calculated 3.34 so selecting UBBR = 3 yields a baud rate of 125kBaud. And the error rate can be calculated as follows: \$e = |1 - (calculated Baud) / (desired Baud)| = |1 - 125k/115.2k|= 8.5\% \$ which is terribly unacceptable. With an 8MHz crystal and assuming maximum allowable \$e = 2\%\$, the maximum allowable baud rate will be 38.4kBaud. So, the slower the baud rate, the lesser the error rate.  Consequently, instead of 8MHz, I recommend you to use 7.3728MHz (1.8432 x 4). Also: How critical are UART frequencies?

Anyway, my suggestions about PCB design:


Use huge GND planes.
Keep tracks as short as possible.
Place the components as close as possible.
Add a GND pad for crystal's outer case. Grounding the case can improve the performance.
Keep positive and GND tracks/planes as close as possible.