Trade in High Liquidity Periods
One of the best ways to minimize spread costs is to trade during periods of high liquidity. Liquidity connotes the capability to buy or sell an asset with ease and without impacting its price; in liquid markets, buyers and sellers are plenty, which means a thin bid-ask spread. This is because effective competition between market participants would attain a better market, thereby pulling prices to be closer.
For instance, major overlaps of the major financial markets, like when the U.S. and European markets are both open, tend to create periods of high liquidity for cryptocurrencies such as Bitcoin and Ethereum. Trading during this period could bring about much lower spread costs than opposed to off-hour trading, like late at night or over the weekend, when liquidity is likely lower and spreads are wider.

Select High Liquidity Pairs
All trading pairs are not the same. For instance, some are more liquid, such as BTC/USD or ETH/USD, in the sense that they exchange hands at high magnitudes. Such pairs will usually have smaller spreads, making them more cost-effective for traders. Less popular pairs, especially those involving minor altcoins, may suffer from poor liquidity, which usually means wider spreads and higher trading costs.
To select the cryptocurrencies that you are going to trade, it would be better to focus on those pairs with a high level of liquidity and tight spreads. That ensures you're not spending too much on spread costs, which really add up over time, especially if you're one of those traders making a lot of trades.

Use Limit Orders Instead of Market Orders
The type of order placed can have a significant influence on the spread cost you will end up incurring. Market orders, which tend to be executed on the spot at the best available price, are typically paying the full spread. This is because market orders source liquidity from the tightest prices on the other side of the order book. Thus, a buyer would be willing to pay the current bid or ask price without any haggling.
On the other hand, limit orders are those in which one can predetermine the price at which one may wish to buy or sell. In this regard, since, in limit orders, one sets a maximum or minimum price that is agreeable to selling or buying, then often, one will not end up paying the whole of the spread. It is especially useful in trading in illiquid or volatile markets, where market orders are costly due to wide spreads.
For instance, if the current bid-ask spread for Ethereum is $10, with a bid of $2,600 and an ask of $2,610, putting in a limit buy order for $2,605 would potentially save you $5 versus executing a market order at $2,610. If such practices are followed over a period of time, the usage of limit orders translates to overall significantly reduced trading costs.

Avoid Trading When Volatile
The market spread, however, may further broaden during times of volatility as market makers incorporate such increased risk in their price adjustments. Traders would then realize that, under such periods, the cost of entering into a position or exiting it is very high due to a wider spread. Such high volatility can be caused by major news events, economic announcements, or just sudden market shifts.
To reduce spread costs, it is wise to avoid trading at such times unless it is absolutely necessary. Rather, these short-lived moments of action could be used on stable trading occasions when spreads can somewhat be expected to be narrow. This approach helps keep your trading costs low and reduces the risk of paying more than expected due to sudden market movements.

Exploit Arbitrage Opportunities
Arbitrage is an approach to taking advantage of price differences for the same assets between different exchanges. Under a spread cost perspective, arbitrage can reduce the cost that one might have paid out for buying an asset on one exchange with a tight spread and selling on another with higher prices. This strategy doesn't just make you profits based on the difference in prices but, in effect, reduces the costs that would have been incurred by this spread.
For example, if Bitcoin trades at $58,000 on Exchange A with a tight spread and $58,050 on Exchange B, where the spread is wider, sell through Exchange A and buy it back on Exchange B, pocketing the $50 difference. That way, you will be able to maximize capturing the $50 difference in a sale price while reducing your exposure on that wider spread on Exchange B.
On the other hand, though arbitrage opportunities can provide one with a source of profitable gains, they have to be efficiently seized by taking into account transaction costs, withdrawal fees, and duration differences in fund transfers between platforms. All these have to be handled and managed with extreme care to make sure that a potential profit covers the cost involved.