How to increase throughput of xbee?
Like Jerome said, your XBee module is probably at its limits... but here are some factors you may still be able to change, and other you probably can't:


Forward Error Correction (FEC, CRC, etc). Higher FEC lowers throughput, but decreases errors.
Transmitter Output power - The higher, the better SNR &amp; Eb/N0 and lower Bit Error Rate (BER), but consumes more power.
Channel noise - the lower, the better SNR &amp; Eb/N0.. again lower BER. Make sure you are transmitting on a clean channel. If you are on 2.4GHz or 5.8GHz, wifi is probably interfering.
Antenna gain - higher gain (more dBi) antennas will give you more EIRP horizontally (but less vertically). I would recommend atleast a 6dBi antenna for 360-horizontal applications. You are allowed a MAX EIRP of 36dBm in the ISM bands (2.4GHz, 900MHz, 5GHz, etc). The higher your EIRP, the longer distance you can transmit.
Modulation Scheme (for example: QPSK is one of the most robust modulation schemes, FHSS is slow, FSK is a bandwidth hog)
Dispersion &amp; Multipath in a channel (OFDM is a better modulation to fight this)
RF Bandwidth - the higher the more baseband bandwidth you can use
Carrier Frequency - 2.4GHz is smaller &amp; cheaper, but very noisy; 900MHz is much better in terms of multipath and noise.


You will probably have to google some of these terms...

Alot of these factors are locked as soon as you purchase a transceiver. Some of these you can change. You need to make sure that you have the best system and settings for your application. 

Without knowing the model of your chipset, my guess is that the only things you probably have control of is:

1, 2, &amp; 4